feat: Konversationsgedaechtnis - Kurzzeit-Gespraechsverlauf pro Session

- Store: messages-Tabelle + append_message/get_recent_messages (mit Mandanten-Schutz)
- LLMProvider.complete um history erweitert (openrouter + local bauen system+history+user)
- Orchestrator.chat_text reicht den Verlauf ans LLM weiter
- chat.py: bei session_id letzte HISTORY_MAX_MESSAGES laden, danach User-/Assistant-Turn speichern
- ohne session_id weiterhin zustandslos; ?debug zeigt history_len
- Config HISTORY_MAX_MESSAGES (Default 10)
- Tests: 32 gruen (3 neue Gedaechtnis-Tests)
- Doku aktualisiert (README, BEDIENUNGSANLEITUNG, Architektur)

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

app/store.py

@@ -70,6 +70,14 @@ class Store(ABC):
     def update_session(self, session_id: str, user_id: str, values: dict) -> Session:
         """Erstellt/aktualisiert eine Session des Nutzers. Fremde Session -> SessionOwnershipError."""
 
+    @abstractmethod
+    def append_message(self, session_id: str, user_id: str, role: str, content: str) -> None:
+        """Haengt eine Nachricht an die Session an. Fremde Session -> SessionOwnershipError."""
+
+    @abstractmethod
+    def get_recent_messages(self, session_id: str, limit: int) -> list[dict]:
+        """Liefert die letzten `limit` Nachrichten chronologisch ([{'role','content'}, ...])."""
+
 
 class SQLiteStore(Store):
     def __init__(self, db_path: str):
@@ -102,6 +110,15 @@ class SQLiteStore(Store):
                     created_at TEXT NOT NULL,
                     updated_at TEXT NOT NULL
                 );
+                CREATE TABLE IF NOT EXISTS messages (
+                    id INTEGER PRIMARY KEY AUTOINCREMENT,
+                    session_id TEXT NOT NULL,
+                    role TEXT NOT NULL,
+                    content TEXT NOT NULL,
+                    created_at TEXT NOT NULL
+                );
+                CREATE INDEX IF NOT EXISTS idx_messages_session
+                    ON messages(session_id, id);
                 """
             )
 
@@ -195,3 +212,35 @@ class SQLiteStore(Store):
                     (session_id, user_id, payload, now, now),
                 )
         return Session(id=session_id, user_id=user_id, data=data)
+
+    # ----- Nachrichten / Gespraechsverlauf ----------------------------------
+    def append_message(self, session_id: str, user_id: str, role: str, content: str) -> None:
+        existing = self.get_session(session_id)
+        if existing and existing.user_id != user_id:
+            raise SessionOwnershipError(
+                f"Session {session_id!r} gehoert einem anderen Nutzer"
+            )
+        now = _now()
+        with self._connect() as conn:
+            if not existing:
+                conn.execute(
+                    "INSERT INTO sessions (id, user_id, data_json, created_at, updated_at)"
+                    " VALUES (?, ?, ?, ?, ?)",
+                    (session_id, user_id, "{}", now, now),
+                )
+            conn.execute(
+                "INSERT INTO messages (session_id, role, content, created_at)"
+                " VALUES (?, ?, ?, ?)",
+                (session_id, role, content, now),
+            )
+
+    def get_recent_messages(self, session_id: str, limit: int) -> list[dict]:
+        if limit <= 0:
+            return []
+        with self._connect() as conn:
+            rows = conn.execute(
+                "SELECT role, content FROM messages WHERE session_id = ?"
+                " ORDER BY id DESC LIMIT ?",
+                (session_id, limit),
+            ).fetchall()
+        return [{"role": row["role"], "content": row["content"]} for row in reversed(rows)]