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/handlers/opds.go

@@ -38,15 +38,15 @@ func NewOPDSHandler(db *database.Queries, libraryService *services.LibraryServic
 	}
 }
 
-// Helper function to get base URL from system config
+// Helper function to get base URL from system config with request-derived fallback
 func (h *OPDSHandler) getBaseURLs(c *echo.Context) (string, string, error) {
-	baseURL, err := h.db.GetSystemConfig(c.Request().Context(), "base_url")
-	if err != nil {
-		return "", "", fmt.Errorf("failed to get base_url from config: %w", err)
+	var dbBaseURL string
+	if config, err := h.db.GetSystemConfig(c.Request().Context(), "base_url"); err == nil {
+		dbBaseURL = config.Value
 	}
-
-	opdsBaseURL := baseURL.Value + "/opds"
-	return baseURL.Value, opdsBaseURL, nil
+	baseURL := deriveBaseURL(c, dbBaseURL)
+	opdsBaseURL := baseURL + "/opds"
+	return baseURL, opdsBaseURL, nil
 }
 
 func (h *OPDSHandler) getAuthToken(c *echo.Context) string {