Split databases and Redis — prepare infrastructure for per-domain isolation

Redis: per-app DB index (0-5) with shared auth DB 15 for SSO keys;
flushdb replaces flushall so deploys don't wipe cross-app auth state.

Postgres: drop 13 cross-domain FK constraints (migration v2t0p8q9r0),
remove dead ORM relationships, add explicit joins for 4 live ones.
Multi-engine sessions (account + federation) ready for per-domain DBs
via DATABASE_URL_ACCOUNT / DATABASE_URL_FEDERATION env vars.

All URLs initially point to the same appdb — zero behaviour change
until split-databases.sh is run to migrate data to per-domain DBs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/events/processor.py

@@ -9,6 +9,9 @@ A dedicated asyncpg LISTEN connection wakes the poll loop immediately when
 emit_activity() fires NOTIFY ap_activity_pending, so latency drops from
 ~2 seconds (poll interval) to sub-100 ms.  The fixed-interval poll remains
 as a safety-net fallback.
+
+The LISTEN connection and poll queries target the federation database
+(DATABASE_URL_FEDERATION) since ap_activities lives there.
 """
 from __future__ import annotations
 
@@ -21,7 +24,7 @@ import asyncpg
 from sqlalchemy import select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from shared.db.session import get_session, DATABASE_URL
+from shared.db.session import get_federation_session, DATABASE_URL_FEDERATION
 from shared.models.federation import APActivity
 from .bus import get_activity_handlers
 
@@ -89,7 +92,7 @@ class EventProcessor:
 
     async def _listen_for_notify(self) -> None:
         """Maintain a LISTEN connection and wake the poll loop on NOTIFY."""
-        dsn = DATABASE_URL.replace("+asyncpg", "")
+        dsn = DATABASE_URL_FEDERATION.replace("+asyncpg", "")
         while self._running:
             try:
                 self._listen_conn = await asyncpg.connect(dsn)
@@ -154,7 +157,7 @@ class EventProcessor:
         """
         cutoff = datetime.now(timezone.utc) - timedelta(seconds=self._stuck_timeout)
         try:
-            async with get_session() as session:
+            async with get_federation_session() as session:
                 filters = [
                     APActivity.process_state == "processing",
                     APActivity.created_at < cutoff,
@@ -180,7 +183,7 @@ class EventProcessor:
     async def _process_batch(self) -> int:
         """Fetch and process a batch of pending activities. Returns count processed."""
         processed = 0
-        async with get_session() as session:
+        async with get_federation_session() as session:
             filters = [
                 APActivity.process_state == "pending",
                 APActivity.process_attempts < APActivity.process_max_attempts,