feat: Tageskontingent und Notfall-Eskalation (#6)

- Quota (app/quota.py): Anfragen pro Nutzer/Tag (usage-Tabelle), DAILY_REQUEST_LIMIT,
  pro Nutzer via prefs uebersteuerbar; 429 (REST) bzw. error-Event (WS); Metrik
- Notfall (app/safety/emergency.py): heuristische Erkennung (de/en); Log im Store
  (emergency_events) + optionaler Webhook (best-effort) + X-Emergency/emergency-Event;
  Metrik emergency_total; Notfaelle umgehen das Kontingent
- verdrahtet in chat/speak/transcribe + WS-Turns
- Tests: 64 gruen (+6); Doku aktualisiert (README, BEDIENUNGSANLEITUNG, Architektur, .env.example)

Hinweis: Notfall-Erkennung ist eine Heuristik (kein Lebensretter); erkannte Texte
sind sensibel -> DSGVO beachten.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/store.py

@@ -97,6 +97,18 @@ class Store(ABC):
     def delete_memory(self, user_id: str, memory_id: int) -> bool:
         """Loescht eine Erinnerung des Nutzers. True, wenn etwas geloescht wurde."""
 
+    @abstractmethod
+    def get_request_count(self, user_id: str, day: str | None = None) -> int:
+        """Anzahl der Anfragen des Nutzers am angegebenen Tag (Default: heute, UTC)."""
+
+    @abstractmethod
+    def add_usage(self, user_id: str, units: int = 0, day: str | None = None) -> int:
+        """Zaehlt eine Anfrage (+units) und liefert die neue Tages-Anfragezahl."""
+
+    @abstractmethod
+    def log_emergency(self, user_id: str, category: str, snippet: str) -> None:
+        """Protokolliert ein erkanntes Notfall-Signal (sensibel!)."""
+
 
 class SQLiteStore(Store):
     def __init__(self, db_path: str):
@@ -146,6 +158,20 @@ class SQLiteStore(Store):
                 );
                 CREATE INDEX IF NOT EXISTS idx_memories_user
                     ON memories(user_id, id);
+                CREATE TABLE IF NOT EXISTS usage (
+                    user_id TEXT NOT NULL,
+                    day TEXT NOT NULL,
+                    requests INTEGER NOT NULL DEFAULT 0,
+                    units INTEGER NOT NULL DEFAULT 0,
+                    PRIMARY KEY (user_id, day)
+                );
+                CREATE TABLE IF NOT EXISTS emergency_events (
+                    id INTEGER PRIMARY KEY AUTOINCREMENT,
+                    user_id TEXT NOT NULL,
+                    category TEXT NOT NULL,
+                    snippet TEXT NOT NULL,
+                    created_at TEXT NOT NULL
+                );
                 """
             )
 
@@ -301,3 +327,41 @@ class SQLiteStore(Store):
                 (memory_id, user_id),
             )
             return cur.rowcount > 0
+
+    # ----- Nutzung / Quota --------------------------------------------------
+    @staticmethod
+    def _today() -> str:
+        return datetime.now(timezone.utc).date().isoformat()
+
+    def get_request_count(self, user_id: str, day: str | None = None) -> int:
+        day = day or self._today()
+        with self._connect() as conn:
+            row = conn.execute(
+                "SELECT requests FROM usage WHERE user_id = ? AND day = ?",
+                (user_id, day),
+            ).fetchone()
+        return int(row["requests"]) if row else 0
+
+    def add_usage(self, user_id: str, units: int = 0, day: str | None = None) -> int:
+        day = day or self._today()
+        with self._connect() as conn:
+            conn.execute(
+                "INSERT INTO usage (user_id, day, requests, units) VALUES (?, ?, 1, ?)"
+                " ON CONFLICT(user_id, day) DO UPDATE SET"
+                " requests = requests + 1, units = units + excluded.units",
+                (user_id, day, units),
+            )
+            row = conn.execute(
+                "SELECT requests FROM usage WHERE user_id = ? AND day = ?",
+                (user_id, day),
+            ).fetchone()
+        return int(row["requests"])
+
+    # ----- Notfall-Protokoll ------------------------------------------------
+    def log_emergency(self, user_id: str, category: str, snippet: str) -> None:
+        with self._connect() as conn:
+            conn.execute(
+                "INSERT INTO emergency_events (user_id, category, snippet, created_at)"
+                " VALUES (?, ?, ?, ?)",
+                (user_id, category, snippet, _now()),
+            )