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

gitea-action/README.md

@@ -1,6 +1,6 @@
 # Deploy via package-updater (composite action)
 
-Notifies `package-updater` to `docker compose pull` + `up -d` for the matching service(s) after a CI build.
+Notifies the `package-updater` master after a CI build. The master records the pushed version on every matching service and, for services in auto mode, updates them and waits for the result.
 
 ## Usage
 
@@ -20,7 +20,7 @@ jobs:
           token: ${{ secrets.UPDATER_TOKEN }}
 ```
 
-`UPDATER_TOKEN` should be set as an organisation-level secret in Gitea so all repos share it.
+`endpoint` is the master's `/update`, and `UPDATER_TOKEN` is the master's `UPDATER_API_KEY`. It should be set as an organisation-level secret in Gitea so all repos share it.
 
 ## Inputs
 
@@ -28,9 +28,20 @@ jobs:
 |---|---|---|---|
 | `endpoint` | yes | — | Full URL to `/update` |
 | `image` | yes | — | Image reference without tag |
-| `tag` | no | `""` | Tag that was just pushed (logged for audit) |
+| `tag` | no | `""` | Tag that was just pushed. Auto services are updated to `<their image repo>:<tag>`, so it must exist in the registry. Omit it to re-pull the tag each service already follows. |
 | `token` | yes | — | Bearer token configured in package-updater |
 
 ## Failure modes
 
-The step exits non-zero if `package-updater` returns HTTP 4xx or 5xx. This is intentional — the workflow surfaces the deploy failure to whoever pushed.
+The step exits non-zero on any HTTP 4xx or 5xx. This is intentional: the workflow surfaces the deploy failure to whoever pushed.
+
+| Code | Meaning |
+|---|---|
+| `200` | Every match was marked, updated, busy or refused. Manual services are only marked, which is not an error. A service that follows no tag is `refused` when the request has no `tag`. |
+| `401` | Wrong or missing token. |
+| `404` | No service uses the image. Check `image` for typos; matching is case-sensitive. |
+| `500` | At least one auto job failed or timed out (`UPDATE_TIMEOUT` on the master). |
+| `503` | The master has no `UPDATER_API_KEY`. |
+
+A request that matches both a working and a failed service now fails as a whole (`500`); the old single-instance updater's `207` no longer exists.
+The master waits for auto jobs, so the request can take minutes; raise any step timeout in your workflow accordingly.