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/config/config_test.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -21,7 +22,7 @@ func setAll(t *testing.T) {
 	for k, v := range required {
 		t.Setenv(k, v)
 	}
-	for _, k := range []string{"PORT", "AGENT_PORT", "AGENT_IMAGE", "LOG_LEVEL"} {
+	for _, k := range []string{"PORT", "AGENT_PORT", "AGENT_IMAGE", "LOG_LEVEL", "UPDATER_API_KEY", "UPDATE_TIMEOUT"} {
 		t.Setenv(k, "")
 	}
 }
@@ -35,6 +36,29 @@ func TestLoad_Defaults(t *testing.T) {
 	assert.Equal(t, "gitea.shcizo.se/shcizo/package-updater:latest", cfg.AgentImage)
 	assert.Equal(t, "info", cfg.LogLevel)
 	assert.Equal(t, "updater-admins", cfg.AllowedGroup)
+	assert.Empty(t, cfg.UpdaterAPIKey)
+	assert.Equal(t, 5*time.Minute, cfg.UpdateTimeout)
+}
+
+func TestLoad_WebhookSettings(t *testing.T) {
+	setAll(t)
+	t.Setenv("UPDATER_API_KEY", "s3cret")
+	t.Setenv("UPDATE_TIMEOUT", "90s")
+	cfg, err := Load()
+	require.NoError(t, err)
+	assert.Equal(t, "s3cret", cfg.UpdaterAPIKey)
+	assert.Equal(t, 90*time.Second, cfg.UpdateTimeout)
+}
+
+func TestLoad_InvalidUpdateTimeout(t *testing.T) {
+	for _, v := range []string{"abc", "0s", "-5m", "30"} {
+		t.Run(v, func(t *testing.T) {
+			setAll(t)
+			t.Setenv("UPDATE_TIMEOUT", v)
+			_, err := Load()
+			assert.ErrorContains(t, err, "UPDATE_TIMEOUT must be a positive duration")
+		})
+	}
 }
 
 func TestLoad_EachRequiredVariable(t *testing.T) {