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

database/schema/schema.sql

@@ -1149,12 +1149,14 @@ CREATE TABLE IF NOT EXISTS system_config (
     updated_by UUID REFERENCES users(id)
 );
 
--- Pre-seeded values
-INSERT INTO system_config (key, value) VALUES
-('base_url', 'https://bookhoard.example.com'),
-('opds_base_url', 'https://bookhoard.example.com/opds'),
-('api_base_url', 'https://bookhoard.example.com/api')
-ON CONFLICT (key) DO NOTHING;
+-- One-time cleanup: clear the old placeholder seed so the startup logic
+-- can re-seed from the BASE_URL env var (or the setup wizard can set it).
+UPDATE system_config SET value = ''
+WHERE key = 'base_url' AND value = 'https://bookhoard.example.com';
+UPDATE system_config SET value = ''
+WHERE key = 'opds_base_url' AND value = 'https://bookhoard.example.com/opds';
+UPDATE system_config SET value = ''
+WHERE key = 'api_base_url' AND value = 'https://bookhoard.example.com/api';
 
 -- Create opds_tokens table (device-specific OPDS access tokens)
 CREATE TABLE IF NOT EXISTS opds_tokens (