refactor(ingest): optimize event chain queries

backend/app/ingestion/service.py

@@ -13,7 +13,7 @@
 import uuid
 from datetime import datetime, timezone
 UTC = timezone.utc
-from typing import Any
+from typing import Any, Optional
 
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session as DBSession
@@ -67,7 +67,7 @@ class IngestService:
     - Append-only storage
     """
 
-    def __init__(self, service_id: str | None = None):
+    def __init__(self, service_id: Optional[str] = None):
         """
         Initialize ingestion service.
 
@@ -100,10 +100,10 @@ def __init__(self, service_id: str | None = None):
 
     def start_session(
         self,
-        session_id_str: str | None = None,
+        session_id_str: Optional[str] = None,
         authority: str = "server",
-        agent_name: str | None = None,
-        user_id: int | None = None,
+        agent_name: Optional[str] = None,
+        user_id: Optional[int] = None,
     ) -> str:
         """
         Start a new session with specified authority.
@@ -142,6 +142,7 @@ def start_session(
                 agent_name=agent_name,
                 user_id=user_id,
                 ingestion_service_id=self.service_id if authority == "server" else None,
+                is_replay=False,
             )
 
             db.add(session)
@@ -157,7 +158,7 @@ def append_events(
         self,
         session_id: str,
         events: list[dict[str, Any]],
-        db: DBSession | None = None,
+        db: Optional[DBSession] = None,
     ) -> dict[str, Any]:
         """
         Append events to session with constitutional validation.
@@ -247,7 +248,8 @@ def append_events(
                 # Store in database
                 event_chain = EventChain(
                     event_id=uuid.UUID(event_envelope["event_id"]),
-                    session_id=session.session_id_str,
+                    session_id=session.id,
+                    session_id_str=session.session_id_str,
                     sequence_number=event_envelope["sequence_number"],
                     timestamp_wall=_parse_wall_timestamp(event_envelope["timestamp_wall"]),
                     timestamp_monotonic=event_data.get("timestamp_monotonic", 0),
@@ -429,7 +431,7 @@ def seal_session(self, session_id: str) -> dict[str, Any]:
 
     # --- Private helper methods ---
 
-    def _get_last_event_hash(self, db: DBSession, session: Session) -> str | None:
+    def _get_last_event_hash(self, db: DBSession, session: Session) -> Optional[str]:
         """Get hash of last event in session, or None if no events."""
         last_event = (
             db.query(EventChain)
@@ -520,8 +522,8 @@ def _emit_log_drop(
         session: Session,
         reason: str,
         dropped_count: int,
-        first_missing_seq: int | None = None,
-        last_missing_seq: int | None = None,
+        first_missing_seq: Optional[int] = None,
+        last_missing_seq: Optional[int] = None,
         owns_session: bool = True,
     ) -> None:
         """