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/storage/db/models.py

@@ -68,6 +68,11 @@ auth_events_table = Table(
     metadata,
     Column("id", UUID(as_uuid=True), primary_key=True, server_default=func.gen_random_uuid()),
     Column("user_id", Text, nullable=False),
+    # Who performed the action, and (when the action is about a user) whom it
+    # was performed on. ``user_id`` predates both and is kept as the per-user
+    # feed key; see migration 0034.
+    Column("actor_id", Text, nullable=False),
+    Column("target_id", Text),
     Column("event", Text, nullable=False),
     Column("ip", Text),
     Column("user_agent", Text),