fix: merge Phase 1 account_id chain with main head to resolve multiple-heads error

Combines the Phase 1 tenant isolation chain (064 → ... → 174f442795b7)
with the main sequential chain (064 → ... → 070) into a single Alembic
head (a9f3b2c1d4e5) so `alembic upgrade head` in the Dockerfile works
without ambiguity.

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

backend/alembic/versions/a9f3b2c1d4e5_merge_phase1_with_main.py

@@ -0,0 +1,24 @@
+"""merge Phase 1 tenant isolation chain with main head
+
+Revision ID: a9f3b2c1d4e5
+Revises: 070, 174f442795b7
+Create Date: 2026-04-09 00:00:00.000000
+
+Merge migration: consolidates the Phase 1 account_id chain (cc214c63aa30 → … → 174f442795b7)
+with the main sequential chain (… → 070) into a single head so that
+`alembic upgrade head` works without ambiguity.
+"""
+from typing import Sequence, Union
+
+revision: str = 'a9f3b2c1d4e5'
+down_revision: Union[str, tuple] = ('070', '174f442795b7')
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    pass
+
+
+def downgrade() -> None:
+    pass