Rewrite the README as highlights with screenshots; move the detail into docs/

The README was 1,240 lines of reference material and still described a
SQLite quick start. It is now a short tour (highlights, screenshots of the
web UI, an accurate quick start against Postgres), and each topic has its
own page under docs/ with an index: deployment, configuration, Alertmanager,
incidents, notifications, escalation, dead man's switches, single sign-on,
web UI, API and development. SERVICE-ACCOUNTS.md is rewritten from a
proposal into a reference, and TEAM-LOOKUP.md is gone with the endpoint it
described. The "Upgrading to ..." sections for an unreleased product are
dropped.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

docs/alertmanager.md

@@ -0,0 +1,62 @@
+# Alertmanager configuration
+
+_Pointing Alertmanager at the server._ Back to the [README](../README.md) and the [documentation index](./README.md).
+
+Alerts arrive on a team's **integration key**, which says both that the sender
+may post and which team the alerts belong to. Mint one as an owner of the team:
+
+```bash
+curl -X POST https://terdut.example.com/api/teams/1/integrations \
+  -H "Authorization: Bearer $TERDUT_API_KEY" \
+  -H 'Content-Type: application/json' \
+  -d '{"name":"prod alertmanager"}'
+```
+
+The response carries the key and the full URL **once**; only a SHA-256 hash is
+stored. Put it in your `alertmanager.yml`:
+
+```yaml
+receivers:
+  - name: terdut
+    webhook_configs:
+      - url: http://terdut-server:8080/api/integrations/<key>/alertmanager
+        send_resolved: true
+
+route:
+  receiver: terdut
+```
+
+The whole URL is a credential, so treat it like one. Alertmanager 0.26 and
+later can read it from a file with `url_file:` instead, which keeps it out of
+your configuration repository:
+
+```yaml
+      - url_file: /etc/alertmanager/secrets/terdut-webhook-url/url
+        send_resolved: true
+```
+
+The webhook endpoint requires no authentication.
+
+If you use the [dead man's switch](./dead-mans-switch.md) — and the default configuration does — give
+the heartbeat a route of its own, because the deadline is only as tight as the interval feeding it:
+
+```yaml
+route:
+  receiver: terdut
+  repeat_interval: 4h
+  routes:
+    - matchers: [ 'alertname = "Watchdog"' ]
+      receiver: terdut
+      group_wait: 0s
+      group_interval: 1m
+      repeat_interval: 1m
+```
+
+That delivers a heartbeat every **2 minutes**, not every minute. Alertmanager only reconsiders a
+group every `group_interval`, and at exactly one elapsed interval `repeat_interval` has not *quite*
+passed, so the send slips to the next tick — equal values give 2×. Two minutes against the 15 minute
+default is seven heartbeats per window, which is the point; use `group_interval: 30s` if you want
+the numbers to mean what they say.
+
+kube-prometheus-stack users get the `Watchdog` alert (`expr: vector(1)`) for free; it just needs
+routing to terdut rather than to `null`.