Add an admin page, and move the behaviour settings into the database

Closes #5. Three of the server's tunables were environment variables,
which meant changing how long an incident waits before being paged again
required editing a chart, merging it and waiting for a reconcile. They
are behaviour rather than infrastructure, and the difference is who needs
to change them and how often.

The split is by who owns the value. What stays in the environment is
where the server is plugged in: the listen address, the DSN, the ntfy URL
and token, the public URL. Those are needed before the database is open
and two of them are credentials -- the settings endpoint reports that
ntfy is configured and that a token is set, and never what either is.

What moves is how it behaves: the notify repeat interval, the stale
window and the archive window. The environment variable becomes the seed
rather than the setting, written once on first start and never
overwritten, so a redeploy cannot put a chart's default back over an
administrator's edit -- the rule the per-team dead man's switches already
follow. The loops read the current value per tick, so a change at 02:00
is obeyed at 02:00.

Key/value rather than a column per knob: #6 and #7 will both add
settings, and a table shaped one-column-per-setting needs a migration for
each. The cost is that values are text and the accessor has to say what
type it wanted, which settings.go does in one place. Unknown keys are
refused rather than stored -- a typo that wrote notify_repeat_second
would otherwise sit in the table looking like configuration and doing
nothing -- and each value has bounds loose enough to catch a slipped
decimal point without having an opinion about anybody's rota.

Disabling an account is new, and is not deleting one. Deleting a user
nulls acknowledged_by and assigned_to, which quietly rewrites who did
what during an incident months after the fact. A disabled user cannot
authenticate by either credential, loses their sessions immediately, and
stays the name on every acknowledgement they made. The check is part of
the lookup in serveAs rather than a test afterwards, so there is no path
where the row is loaded and the flag is then forgotten.

The page itself is a fourth tab, shown only to an administrator and only
as a courtesy: every endpoint under it is refused with 403 regardless, so
somebody who types /admin gets an explanation rather than a blank screen.
It lists teams with their size and open-incident count, users with their
flags, and the settings with their bounds -- plus the environment half,
read-only, so somebody hunting for the ntfy URL learns where it lives
instead of concluding the server has none.

Delete is disabled rather than offered-and-refused for a team with open
incidents, and neither admin action is offered on your own account, since
the server refuses both.

Claude-Session: https://claude.ai/code/session_01RHPj4ggeFdEjKKfm4SHbD7

README.md

@@ -155,12 +155,25 @@ somewhere to exec. The sidecar, the PVC and the `backupSidecar` values are all g
 
 ## Configuration
 
+Two kinds of setting, split by who changes them and how often.
+
+**Where the server is plugged in** stays in the environment: the listen address,
+the database DSN, the ntfy URL and token, the public URL. They are needed before
+the database is open, and two of them are credentials.
+
+**How the server behaves** lives in the database and is edited by an
+administrator in the web UI or through `PUT /api/admin/settings`, taking effect
+on the next sweep rather than at the next restart. The variables below marked
+**seed** are the value each of those starts from: written once, on first start,
+and never overwritten afterwards — a redeploy cannot put a chart's default back
+over an administrator's edit.
+
 | Variable | Default | Description |
 |---|---|---|
 | `TERDUT_ADDR` | `:8080` | TCP address to listen on |
 | `TERDUT_DB_DSN` | — | **Required.** Postgres connection string, e.g. `postgres://terdut:secret@localhost:5432/terdut?sslmode=require` |
-| `TERDUT_ARCHIVE_AFTER` | `168h` (7d) | How long a resolved alert or incident stays in the default list before being auto-archived |
-| `TERDUT_STALE_AFTER` | `6h` | How long a firing alert may go without a refreshing webhook before it is treated as resolved — **must exceed your Alertmanager `repeat_interval`** |
+| `TERDUT_ARCHIVE_AFTER` | `168h` (7d) | **seed.** How long a resolved alert or incident stays in the default list before being auto-archived |
+| `TERDUT_STALE_AFTER` | `6h` | **seed.** How long a firing alert may go without a refreshing webhook before it is treated as resolved — **must exceed your Alertmanager `repeat_interval`** |
 | `TERDUT_DEADMAN_MATCHERS` | `alertname=Watchdog` | The **default** matchers a team starts with — switches are per team now, and this seeds teams that have no configuration of their own. `;` separates matchers, `,` the label conditions within one, `=` is exact equality. Every matcher must name an `alertname` |
 | `TERDUT_DEADMAN_TIMEOUT` | `15m` | How long a heartbeat may go unheard before its switch is declared dead — **must be shorter than the `repeat_interval` of the route carrying it**. `0` disables dead man's switch handling |
 | `TERDUT_DEADMAN_SEVERITY` | `critical` | Severity a dead man's switch incident opens at |
@@ -168,7 +181,7 @@ somewhere to exec. The sidecar, the PVC and the `backupSidecar` values are all g
 | `TERDUT_NTFY_TOKEN` | — | Bearer token for an access-controlled ntfy |
 | `TERDUT_NTFY_FALLBACK_TOPIC` | — | Topic used when nobody is on call |
 | `TERDUT_PUBLIC_URL` | — | Base URL a phone uses to reach this server: the notification's link into the web UI, its Acknowledge button, and whether the session cookie is `Secure` |
-| `TERDUT_NOTIFY_REPEAT` | `15m` | How long an incident may sit unacknowledged before it is paged again. `0` notifies once and never repeats |
+| `TERDUT_NOTIFY_REPEAT` | `15m` | **seed.** How long an incident may sit unacknowledged before it is paged again. `0` notifies once and never repeats |
 
 Durations use Go syntax (`30m`, `12h`, `168h`). An unparseable value falls back to the default.
 
@@ -522,11 +535,20 @@ on anybody's.
 | `POST` | `/api/users` | **admin** | Create user `{"username","email"}`. Not an administrator |
 | `DELETE` | `/api/users/{id}` | **admin** | Delete user (cascades to keys). `409` for yourself or the last administrator |
 | `PUT` | `/api/users/{id}/admin` | **admin** | Grant or revoke the administrator flag `{"is_admin"}`. `409` for yourself or the last administrator |
+| `PUT` | `/api/users/{id}/disabled` | **admin** | Take an account out of use, or put it back `{"disabled"}`. `409` for yourself or the last administrator |
 | `PUT` | `/api/users/{id}/notify` | self or admin | Set push notification target `{"ntfy_topic"}` — empty string clears it |
 | `PUT` | `/api/users/{id}/password` | self or admin | Set web UI password `{"password","current_password"}`. `current_password` is required only when changing your own existing password. Ends the user's other sessions |
 | `POST` | `/api/users/{id}/api-keys` | self or admin | Issue API key `{"name"}` — key shown once |
 | `DELETE` | `/api/users/{id}/api-keys/{keyID}` | self or admin | Revoke API key |
 
+### Administration
+
+| Method | Path | Who | Description |
+|---|---|---|---|
+| `GET` | `/api/admin/teams` | **admin** | Every team on the server, with its member and open-incident counts. `/api/teams` answers "what am I in"; this answers "what is there" |
+| `GET` | `/api/admin/settings` | **admin** | The editable settings with their bounds, plus the environment-configured ones, read-only. Never credentials |
+| `PUT` | `/api/admin/settings` | **admin** | Change one or more `{"key": seconds}`. `400` for an unknown key or a value outside its bounds |
+
 ### Alert ingestion
 
 Alerts arrive on a team's integration key. The key is both the credential and the
@@ -548,6 +570,7 @@ and was removed in v0.13.0 once senders had moved onto keys.
 |---|---|---|---|
 | `GET` | `/api/teams` | any | The caller's own teams, each with their role |
 | `POST` | `/api/teams` | any | Create a team `{"name"}`; the creator becomes its first owner |
+| `PUT` | `/api/teams/{teamID}` | **owner** | Rename it `{"name"}`. `409` if the name is taken |
 | `DELETE` | `/api/teams/{teamID}` | **owner** | Delete a team and everything under it. `409` while it has open incidents |
 | `GET` | `/api/teams/{teamID}/members` | member | Who is in the team |
 | `POST` | `/api/teams/{teamID}/members` | **owner** | Add a member, or change their role `{"user_id","role"}` |