fix: OPDS base_url placeholder bug + setup gate requires base_url

Three bugs fixed:

1. Schema seeded base_url with fake placeholder 'bookhoard.example.com'.
   Removed seed; startup now seeds from BASE_URL env var only if DB row
   is empty (admin changes persist across restarts). One-time UPDATE
   clears the placeholder in existing installs.

2. config.GetBaseURL() had a broken type assertion (local SystemConfigRow
   vs database.SystemConfig) that always failed, returning . Admin panel
   showed env var fallback instead of actual DB value. Fixed with a
   function-type getter that properly wraps the DB query.

3. OPDS handler read base_url only from DB with no fallback. When DB had
   the placeholder, all feed links pointed to an unreachable domain,
   breaking KOReader search/download. Added deriveBaseURL() helper that
   falls back to the request Host/scheme when DB value is empty.

Setup gate improvements:
- isSetupComplete now requires both admin user AND non-empty base_url
- Setup middleware no longer exempts all /api/ routes; only allows
  /api/auth/register, /api/auth/login, /api/system/config before setup
  is complete. All other API routes get 503.
- Cache invalidated when base_url is saved via admin settings

Dev workflow:
- New bruno/NewDevDBSetup/SetBaseUrl.yml for dev DB setup
- NewDB.sh runs SetBaseUrl between RegisterUser and CreateEbookLibrary

internal/router/router.go

@@ -72,6 +72,24 @@ type Config struct {
 	LibraryService          *services.LibraryService
 }
 
+// getBaseURL returns the configured base URL from the database, falling back to
+// the env var / config default. Uses a closure to adapt the database query to
+// config.SystemConfigGetter.
+func (cfg *Config) getBaseURL(ctx context.Context) string {
+	getter := func(ctx context.Context, key string) (string, error) {
+		row, err := cfg.Queries.GetSystemConfig(ctx, key)
+		if err != nil {
+			return "", err
+		}
+		return row.Value, nil
+	}
+	baseURL := config.GetBaseURL(ctx, getter)
+	if baseURL == "" {
+		baseURL = cfg.Cfg.BaseURL
+	}
+	return baseURL
+}
+
 // createJWTMiddleware creates a JWT middleware with proper user context setup
 func createJWTMiddleware(cfg *Config) echo.MiddlewareFunc {
 	return echojwt.WithConfig(echojwt.Config{