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>

src/fabledassistant/routes/admin.py

@@ -195,6 +195,10 @@ async def get_base_url_setting():
 async def update_base_url():
     data = await request.get_json()
     url = (data.get("base_url") or "").strip().rstrip("/")
+    if url:
+        scheme = url.split("://")[0].lower() if "://" in url else ""
+        if scheme not in ("http", "https"):
+            return jsonify({"error": "Base URL must use http or https"}), 400
     uid = get_current_user_id()
     await set_setting(uid, "base_url", url)
     await log_audit("base_url_config", user_id=uid, username=g.user.username, ip_address=request.remote_addr, details={"base_url": url})