fix(settings): audit pass — model auto-pull on startup, background_model empty-string bug, base URL validation

- Startup now pulls Config.OLLAMA_MODEL (system default chat model) — previously only
  embedding and background models were pulled; the primary chat model was skipped
- _warm_user_models expanded to also pull user-configured default_model and
  background_model overrides that are missing from Ollama, rather than logging and
  skipping them; pulls run before warm/KV-cache priming
- Add background_model to _MODEL_KEYS in settings route so clearing the dropdown
  deletes the DB row instead of saving "", which caused Ollama failures in tag
  suggestions, title generation, project summaries, and RSS classification
- Add http/https scheme validation to PUT /api/admin/base-url matching the CalDAV
  route pattern; a bad value no longer silently breaks invite/password-reset links
- Update admin voice config description: "Reload models" button exists to avoid
  a server restart, so the old "restart required" text was misleading

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -2564,7 +2564,7 @@ FABLE_API_KEY=&lt;your-api-key&gt;</pre>
         <h2>Voice (Speech-to-Speech)</h2>
         <p class="section-desc">
           Enable self-hosted voice using faster-whisper (STT) and Kokoro (TTS).
-          Models load at startup — a server restart is required after changing these settings.
+          Save settings then click "Reload models" to apply without restarting the server.
           Install dependencies first: <code>pip install faster-whisper kokoro soundfile</code>
         </p>
         <div class="checkbox-field">