Scope everything to a team, and route alerts by integration key

The core of #4, and what #1 is for: terdut stops being one shared space.
A team owns its incidents, alerts, schedule and integrations; a user sees
exactly the teams they are in. Everything that existed moves into one
Default team and every existing user becomes an owner of it, so the
upgrade is a no-op for the people using it.

Ingestion is the load-bearing half. An alert arrives on a team's
integration key, and the key is both the credential and the routing: it
says that the sender may post, and which team the alerts belong to. That
also closes the unauthenticated webhook -- the old path stays for one
release, deprecated and routed to the oldest team, so an upgrade does not
stop delivering while somebody edits the Alertmanager config.

Scoping is enforced in as few places as possible, because the failure
mode is silent. serveAs loads the caller's memberships once; list queries
carry `team_id = ANY(...)`; and every incident route goes through
incidentIDParam, which now parses the id AND checks the team in the same
call, so a new handler cannot remember the first half and forget the
second. Anything in another team is 404, never 403: whether an incident
exists is that team's business.

Two bugs this found, both of which would have been silent:

  * upsertAlerts decided "is this a new occurrence" by looking up the
    fingerprint alone. Across teams that made team B's first alert look
    like a re-send of team A's, so it opened no incident at all. The
    lookups are keyed on (team_id, fingerprint) now, as the index is.

  * Every uniqueness rule was written for one tenant. Two teams watching
    two clusters legitimately see the same fingerprint, the same
    groupKey, and want somebody on call on the same day; all three
    constraints move to include team_id.

Roles inside a team are separate from the system administrator flag: an
owner configures the team, a member works its incidents, and an admin is
NOT implicitly in every team -- administration is about accounts, not
about reading other people's incidents. An admin can still repair a team
whose owner has left, which is why requireTeamOwner lets them through.

A shift can only be given to somebody in the team. Paging a person who
cannot open the incident is worse than paging nobody.

The UI is updated only as far as keeping it working: it loads the
viewer's teams with the session and uses the first one, since nobody has
a second yet. "On call now" shows every team the viewer is in, named only
when there is more than one, so the common case reads exactly as before.
The team switcher, badges and per-team settings pages are the next step.

Breaking for API clients: the schedule endpoints moved under the team,
and /api/schedule/current returns an array rather than an object or a
404. terdut-tui will need a version for that.

Per-team dead-man configuration is deliberately not here. A heartbeat's
incident already opens in the team whose key received it, which is the
part that matters for isolation; moving the matchers out of env into
per-team rows is a change to how deadman.go is configured rather than to
who sees what.

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

internal/api/deadman.go

@@ -171,6 +171,7 @@ func ParseDeadmanConfig(matchers string, timeout time.Duration, severity string)
 // deadmanAlert is one switch: the alert row carrying its last heartbeat.
 type deadmanAlert struct {
 	id          int64
+	teamID      int64
 	fingerprint string
 	labels      map[string]string
 	matcher     DeadmanMatcher
@@ -236,7 +237,7 @@ func deadmanAlerts(ctx context.Context, db *sql.DB, cfg DeadmanConfig) ([]deadma
 	}
 
 	rows, err := db.QueryContext(ctx, `
-		SELECT id, fingerprint, labels, status, received_at
+		SELECT id, team_id, fingerprint, labels, status, received_at
 		FROM   alerts
 		WHERE  name IN (`+args.addList(nameList)+`)
 		  AND  archived_at IS NULL`, args.all()...)
@@ -249,7 +250,7 @@ func deadmanAlerts(ctx context.Context, db *sql.DB, cfg DeadmanConfig) ([]deadma
 	for rows.Next() {
 		var a deadmanAlert
 		var labelsJSON, status string
-		if err := rows.Scan(&a.id, &a.fingerprint, &labelsJSON, &status, &a.receivedAt); err != nil {
+		if err := rows.Scan(&a.id, &a.teamID, &a.fingerprint, &labelsJSON, &status, &a.receivedAt); err != nil {
 			return nil, err
 		}
 		json.Unmarshal([]byte(labelsJSON), &a.labels) //nolint:errcheck
@@ -280,8 +281,8 @@ func deadmanDied(ctx context.Context, db *sql.DB, cfg DeadmanConfig, notify Noti
 	if err := db.QueryRowContext(ctx, `
 		SELECT COALESCE(MAX(triggered_at), 0),
 		       COUNT(*) FILTER (WHERE resolved_at IS NULL)
-		FROM   incidents WHERE group_key = $1`,
-		sw.groupKey()).Scan(&lastTriggered, &open); err != nil {
+		FROM   incidents WHERE team_id = $1 AND group_key = $2`,
+		sw.teamID, sw.groupKey()).Scan(&lastTriggered, &open); err != nil {
 		return err
 	}
 	if open > 0 || sw.receivedAt <= lastTriggered {
@@ -314,7 +315,9 @@ func deadmanDied(ctx context.Context, db *sql.DB, cfg DeadmanConfig, notify Noti
 		sev = &severity
 	}
 
-	incidentID, err := openIncident(ctx, tx, notify, sw.groupKey(),
+	// The incident opens in the team whose integration received the heartbeat:
+	// the switch belongs to whoever is watching that source, not to the install.
+	incidentID, err := openIncident(ctx, tx, notify, sw.teamID, sw.groupKey(),
 		"No heartbeat from "+sw.matcher.String(), sw.labels, sev)
 	if err != nil {
 		return err
@@ -343,7 +346,8 @@ func deadmanRecovered(ctx context.Context, db *sql.DB, sw deadmanAlert) error {
 	var incidentID int64
 	switch err := db.QueryRowContext(ctx, `
 		SELECT id FROM incidents
-		 WHERE group_key = $1 AND resolved_at IS NULL`, sw.groupKey()).Scan(&incidentID); {
+		 WHERE team_id = $1 AND group_key = $2 AND resolved_at IS NULL`,
+		sw.teamID, sw.groupKey()).Scan(&incidentID); {
 	case err == sql.ErrNoRows:
 		return nil
 	case err != nil: