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/frontend.go

@@ -9,7 +9,6 @@ import (
 	"strconv"
 	"time"
 
-	"bookhoard/internal/config"
 	"bookhoard/internal/database"
 	"bookhoard/internal/handlers"
 	"bookhoard/internal/services"
@@ -734,10 +733,7 @@ func registerFrontendRoutes(cfg *Config) {
 		}
 
 		// Get base URL from database config with fallback to config/env var
-		baseURL := config.GetBaseURL(c.Request().Context(), cfg.Queries)
-		if baseURL == "" {
-			baseURL = cfg.Cfg.BaseURL
-		}
+		baseURL := cfg.getBaseURL(c.Request().Context())
 
 		var buf bytes.Buffer
 		err = templates.Devices(user, devices, pendingList, errorMsg, baseURL).Render(c.Request().Context(), &buf)
@@ -995,10 +991,7 @@ func registerFrontendRoutes(cfg *Config) {
 		}
 
 		// Fetch current system configuration - just base_url
-		baseURL := config.GetBaseURL(c.Request().Context(), cfg.Queries)
-		if baseURL == "" {
-			baseURL = cfg.Cfg.BaseURL
-		}
+		baseURL := cfg.getBaseURL(c.Request().Context())
 
 		systemConfig := map[string]string{
 			"base_url":         baseURL,
@@ -1086,10 +1079,7 @@ func registerFrontendRoutes(cfg *Config) {
 		}
 
 		// Get base URL from database config with fallback to config/env var
-		baseURL := config.GetBaseURL(c.Request().Context(), cfg.Queries)
-		if baseURL == "" {
-			baseURL = cfg.Cfg.BaseURL
-		}
+		baseURL := cfg.getBaseURL(c.Request().Context())
 
 		var buf bytes.Buffer
 		err = templates.Devices(user, devices, pendingList, errorMsg, baseURL).Render(c.Request().Context(), &buf)