feat: Phase 1 Group 5 — add account_id to PSA and notification tables

psa_post_log: backfill via psa_connection, fallback to posted_by user
psa_member_mappings: backfill via psa_connection
notification_logs: backfill via notification_config

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

backend/app/models/psa_member_mapping.py

@@ -25,6 +25,12 @@ class PsaMemberMapping(Base):
         nullable=False,
         index=True,
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     user_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("users.id", ondelete="CASCADE"),