feat(db): add session_profile table for keystroke-dynamics fingerprints

New purpose-built table with schema_version column committed from day one
so V2 federation gossip can cluster sessions across operators without
retrofitting. Ships with the empty write path (upsert_session_profile);
ingestion of keystroke features (IKI moments, control-char rates, digraph
SimHash) is tracked as V2 work.

Closes gap #2 from SIGNAL_CAPTURE_AUDIT.md.

decnet/web/db/repository.py

@@ -160,6 +160,16 @@ class BaseRepository(ABC):
         """Bulk-fetch behavior rows keyed by attacker IP (JOIN to attackers)."""
         pass
 
+    @abstractmethod
+    async def upsert_session_profile(self, sid: str, data: dict[str, Any]) -> None:
+        """Insert or update the keystroke-dynamics profile row for a session."""
+        pass
+
+    @abstractmethod
+    async def get_session_profile(self, sid: str) -> Optional[dict[str, Any]]:
+        """Retrieve the keystroke-dynamics profile row for a session."""
+        pass
+
     @abstractmethod
     async def get_attacker_by_uuid(self, uuid: str) -> Optional[dict[str, Any]]:
         """Retrieve a single attacker profile by UUID."""