feat: add audit log for feature usage tracking

hindsight-api-slim/hindsight_api/alembic/versions/c2d3e4f5g6h7_add_audit_log_table.py

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+"""Add audit_log table for feature usage tracking.
+
+Merge migration that combines the two existing heads (a3b4c5d6e7f8 + c8e5f2a3b4d1).
+
+Stores raw request/response as JSONB for expandability without future migrations.
+The metadata JSONB column allows adding arbitrary fields in the future.
+
+Revision ID: c2d3e4f5g6h7
+Revises: a3b4c5d6e7f8, c8e5f2a3b4d1
+Create Date: 2026-03-26
+"""
+
+from collections.abc import Sequence
+
+from alembic import context, op
+
+revision: str = "c2d3e4f5g6h7"
+down_revision: str | Sequence[str] | None = ("a3b4c5d6e7f8", "c8e5f2a3b4d1")
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def _get_schema_prefix() -> str:
+    """Get schema prefix for table names (required for multi-tenant support)."""
+    schema = context.config.get_main_option("target_schema")
+    return f'"{schema}".' if schema else ""
+
+
+def upgrade() -> None:
+    schema = _get_schema_prefix()
+
+    op.execute(
+        f"""
+        CREATE TABLE IF NOT EXISTS {schema}audit_log (
+            id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+            action TEXT NOT NULL,
+            transport TEXT NOT NULL,
+            bank_id TEXT,
+            started_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+            ended_at TIMESTAMPTZ,
+            request JSONB,
+            response JSONB,
+            metadata JSONB DEFAULT '{{}}'::jsonb
+        )
+        """
+    )
+
+    op.execute(
+        f"CREATE INDEX IF NOT EXISTS idx_audit_log_action_started ON {schema}audit_log (action, started_at DESC)"
+    )
+    op.execute(f"CREATE INDEX IF NOT EXISTS idx_audit_log_bank_started ON {schema}audit_log (bank_id, started_at DESC)")
+    op.execute(f"CREATE INDEX IF NOT EXISTS idx_audit_log_started ON {schema}audit_log (started_at DESC)")
+
+
+def downgrade() -> None:
+    schema = _get_schema_prefix()
+
+    op.execute(f"DROP INDEX IF EXISTS {schema}idx_audit_log_started")
+    op.execute(f"DROP INDEX IF EXISTS {schema}idx_audit_log_bank_started")
+    op.execute(f"DROP INDEX IF EXISTS {schema}idx_audit_log_action_started")
+    op.execute(f"DROP TABLE IF EXISTS {schema}audit_log")