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

bruno/NewDB.sh

@@ -1,3 +1,3 @@
 #!/bin/bash
 
-bru run --env Bookhoard --delay 500 "NewDevDBSetup/RegisterUser.yml" "NewDevDBSetup/CreateEbookLibrary.yml" "NewDevDBSetup/CreateComicLibrary.yml" "NewDevDBSetup/CreateMangaLibrary.yml" "NewDevDBSetup/AddEbookLibraryFolder.yml" "NewDevDBSetup/AddComicLibraryFolder.yml" "NewDevDBSetup/AddMangaLibraryFolder.yml" "NewDevDBSetup/ScanAllLibraries.yml"
+bru run --env Bookhoard --delay 500 "NewDevDBSetup/RegisterUser.yml" "NewDevDBSetup/SetBaseUrl.yml" "NewDevDBSetup/CreateEbookLibrary.yml" "NewDevDBSetup/CreateComicLibrary.yml" "NewDevDBSetup/CreateMangaLibrary.yml" "NewDevDBSetup/AddEbookLibraryFolder.yml" "NewDevDBSetup/AddComicLibraryFolder.yml" "NewDevDBSetup/AddMangaLibraryFolder.yml" "NewDevDBSetup/ScanAllLibraries.yml"

bruno/NewDevDBSetup/SetBaseUrl.yml

@@ -0,0 +1,38 @@
+info:
+  name: SetBaseUrl
+  type: http
+  seq: 3
+
+http:
+  method: PUT
+  url: '{{base_url}}/api/system/config'
+  auth: inherit
+  body:
+    type: json
+    jsonBody: |-
+      {
+        "base_url": "http://localhost:8765"
+      }
+  headers:
+  - key: Authorization
+    value: Bearer {{token}}
+  - key: Content-Type
+    value: application/json
+
+settings:
+  encodeUrl: true
+  timeout: 0
+  followRedirects: true
+  maxRedirects: 5
+
+docs: |-
+  ## Set Base URL
+
+  Configures the server's base_url during initial dev database setup.
+
+  Must be run after RegisterUser (which provides the auth token) and before
+  any library/device creation (which require setup to be complete).
+
+  **Method:** PUT
+  **Endpoint:** /api/system/config
+  **Auth:** Bearer token (from RegisterUser)