feat(admin): Phase 3 — Laufzeit-Konfiguration ohne Server-Neustart

Neue Tabelle config_overrides in SQLite; RuntimeSettings-Wrapper liest
überschreibbare Felder mit 30s TTL-Cache aus der DB und fällt auf
.env-Werte zurück. dependencies.py und quota.py nutzen runtime_settings
als Default statt des statischen Settings-Singletons.

16 Felder überschreibbar: STT/LLM/TTS-Provider, LLM-Modelle, Stimmen,
Systemprompt, Temperatur, Tageskontingent, Normalisierung u.a.

Backend: GET/PUT/DELETE /api/admin/config/{key}
Admin-UI: neuer Tab "⚙ Einstellungen" mit Inline-Edit und Reset.

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

app/store.py

@@ -152,6 +152,15 @@ class Store(ABC):
     @abstractmethod
     def get_all_usage(self) -> list[dict]: ...
 
+    @abstractmethod
+    def get_config_overrides(self) -> dict[str, str]: ...
+
+    @abstractmethod
+    def set_config_override(self, key: str, value: str) -> None: ...
+
+    @abstractmethod
+    def delete_config_override(self, key: str) -> bool: ...
+
 
 class SQLiteStore(Store):
     def __init__(self, db_path: str):
@@ -216,6 +225,11 @@ class SQLiteStore(Store):
                     snippet TEXT NOT NULL,
                     created_at TEXT NOT NULL
                 );
+                CREATE TABLE IF NOT EXISTS config_overrides (
+                    key TEXT PRIMARY KEY,
+                    value TEXT NOT NULL,
+                    updated_at TEXT NOT NULL
+                );
                 """
             )
             # Migration fuer bestehende DBs: external_id ergaenzen (falls noch nicht da).
@@ -551,3 +565,21 @@ class SQLiteStore(Store):
                 " GROUP BY u.user_id ORDER BY total_requests DESC",
             ).fetchall()
         return [dict(r) for r in rows]
+
+    def get_config_overrides(self) -> dict[str, str]:
+        with self._connect() as conn:
+            rows = conn.execute("SELECT key, value FROM config_overrides").fetchall()
+        return {r["key"]: r["value"] for r in rows}
+
+    def set_config_override(self, key: str, value: str) -> None:
+        with self._connect() as conn:
+            conn.execute(
+                "INSERT INTO config_overrides(key, value, updated_at) VALUES(?,?,?)"
+                " ON CONFLICT(key) DO UPDATE SET value=excluded.value, updated_at=excluded.updated_at",
+                (key, value, _now()),
+            )
+
+    def delete_config_override(self, key: str) -> bool:
+        with self._connect() as conn:
+            cur = conn.execute("DELETE FROM config_overrides WHERE key=?", (key,))
+        return cur.rowcount > 0