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.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"time"
 )
 
 // Config holds the master's runtime configuration.
@@ -20,8 +21,18 @@ type Config struct {
 	Port             string
 	AgentPort        string
 	LogLevel         string
+
+	// UpdaterAPIKey is the bearer token for POST /update (UPDATER_API_KEY).
+	// Empty disables the webhook.
+	UpdaterAPIKey string
+	// UpdateTimeout bounds how long one webhook request waits for its auto
+	// jobs (UPDATE_TIMEOUT, default 5m).
+	UpdateTimeout time.Duration
 }
 
+// defaultUpdateTimeout is used when UPDATE_TIMEOUT is unset.
+const defaultUpdateTimeout = 5 * time.Minute
+
 // Load reads and validates the master configuration, failing fast on
 // anything missing so a half-configured master never starts.
 func Load() (*Config, error) {
@@ -37,6 +48,8 @@ func Load() (*Config, error) {
 		Port:             getenvDefault("PORT", "8080"),
 		AgentPort:        getenvDefault("AGENT_PORT", "8090"),
 		LogLevel:         getenvDefault("LOG_LEVEL", "info"),
+		UpdaterAPIKey:    os.Getenv("UPDATER_API_KEY"),
+		UpdateTimeout:    defaultUpdateTimeout,
 	}
 	for _, r := range []struct{ name, value string }{
 		{"DATABASE_URL", cfg.DatabaseURL},
@@ -57,6 +70,13 @@ func Load() (*Config, error) {
 	if !strings.HasPrefix(cfg.AgentURL, "wss://") {
 		return nil, fmt.Errorf("AGENT_URL must start with wss://")
 	}
+	if v := os.Getenv("UPDATE_TIMEOUT"); v != "" {
+		d, err := time.ParseDuration(v)
+		if err != nil || d <= 0 {
+			return nil, fmt.Errorf("UPDATE_TIMEOUT must be a positive duration such as 5m, got %q", v)
+		}
+		cfg.UpdateTimeout = d
+	}
 	return cfg, nil
 }