feat(audit): split actor from subject on auth events

auth_events.user_id was overloaded and meant something different per call
site: admin mutations filed the row under the target user and hid the acting
admin in metadata->>'by', team events filed it under the actor, and quota
events switched between the two depending on scope. The practical consequence
was that "show me everything admin X did" had no answer.

Adds actor_id (never NULL) and target_id (NULL when the event is not about a
user) alongside the existing column, which keeps its meaning as the per-user
feed key so nothing historical is rewritten. The backfill recovers the actor
from metadata for rows that predate the columns.

Also adds the indexes the cross-user admin feed needs. The only index was
(user_id, created_at DESC), so the global feed -- which orders by created_at
with no user predicate -- fell back to a sequential scan plus a sort on every
page.

The feed gains actor, multi-event, until and free-text search filters, plus a
distinct-event catalogue so the UI can offer what the instance recorded
instead of asking operators to type a name from memory.

docsgpt/api/admin/quotas.py

@@ -119,6 +119,10 @@ def _audit(conn, event: str, scope: str, subject_id: Optional[str], detail: dict
         ip=request.remote_addr,
         user_agent=request.headers.get("User-Agent"),
         metadata={"by": actor, "via": "admin_api", "scope": scope, "subject_id": subject_id, **detail},
+        actor_id=actor or "unknown",
+        # Only a user-scoped policy targets a user; instance and team policies
+        # change configuration, not an account.
+        target_id=subject_id if scope == "user" else None,
     )