feat: master/agent phase 3 — webhook and auto mode (#8) (#22)

## Summary
- `POST /update` on the master (outside OIDC, bearer `UPDATER_API_KEY`; unset → 503). Matches `image` against `image_repo` on all agents, marks `available_*` on every match, starts webhook-triggered jobs for `auto` services targeting `<image_repo>:<tag>` (followed tag when no tag is given), and waits for them up to `UPDATE_TIMEOUT` (default 5m). 404 on no match, 500 if any auto job failed or timed out, otherwise 200.
- Manual/auto toggle per service on the overview and service page (`POST /services/{id}/mode`, CSRF); auto is blocked for self, `build:` and container services with the reason shown. "Update available" badge and an "Available" row.
- `FinishJob` clears `available_*` only when the successful job was created for that exact ref, after it was reported. Migration `0003_webhook.sql`, phase-3 plan doc, README / gitea-action README / CLAUDE.md / deploy example updated. `action.yml` unchanged.

Closes #8

## Behaviour changes for CI
- Mixed results now return 500 (the old 207 is gone); no match returns 404 instead of 200.
- With a `tag`, Compose services get pinned to `<image_repo>:<tag>` via `compose.override.yaml`; the tag must be a pushed tag.
- `busy` (job already active) and `refused` (not updatable) are reported but return 200.
- The reverse proxy read timeout must exceed `UPDATE_TIMEOUT`.

## Test plan
- [x] `go test ./...` and `go test -race ./internal/agent/... ./internal/master/... ./internal/e2e/`
- [x] `go test -tags integration ./internal/master/store/` against throwaway Postgres
- [x] UI verified in browser (mode toggle, disabled reason, badge via SSE, Available row)
- [ ] Manual: webhook with manual service → `marked` + badge
- [ ] Manual: webhook with auto service → waits, `succeeded`, badge cleared, override pinned
- [ ] Manual: wrong token 401, unknown image 404, offline agent 500
- [ ] Gitea action pointed at the master's `/update` in a real CI run

Reviewed-on: #22

internal/master/web/views.go

@@ -11,12 +11,20 @@ import (
 
 type serviceGroup struct {
 	Title    string
-	Services []model.Service
+	Services []serviceRow
+}
+
+// serviceRow is a service as shown in the overview table.
+type serviceRow struct {
+	model.Service
+	AutoBlocked string // why the service cannot be set to auto; "" when it can
 }
 
 // groupServices groups machine-bound services per agent and Swarm services
 // in one group, keeping the store's order (machines first, Swarm last).
-func groupServices(services []model.Service) []serviceGroup {
+// autoBlocked is called for every service and returns the reason it cannot be
+// set to auto, or "" when it can.
+func groupServices(services []model.Service, autoBlocked func(model.Service) string) []serviceGroup {
 	var groups []serviceGroup
 	index := map[string]int{}
 	for _, s := range services {
@@ -32,7 +40,7 @@ func groupServices(services []model.Service) []serviceGroup {
 			index[title] = i
 			groups = append(groups, serviceGroup{Title: title})
 		}
-		groups[i].Services = append(groups[i].Services, s)
+		groups[i].Services = append(groups[i].Services, serviceRow{Service: s, AutoBlocked: autoBlocked(s)})
 	}
 	return groups
 }