fix(audit): chunk + self-resume library scans (stop the 2h queue-hog timeouts)

scan_library_for_rule ran one 2-hour pass that timed out on large libraries and
held the concurrency-1 maintenance queue the whole time, starving vacuum/backup/
normalize (operator-flagged — it was the dominant entry in the 24h failures).

It now runs ~10-min chunks and re-enqueues itself until the library is
exhausted, matching the operator's preferred pattern (reasonable timeout → retry
queued → other things process between). New columns (alembic 0039):
resume_after_id persists the keyset cursor so a chunk continues where the last
left off; last_progress_at lets the recovery sweep tell a progressing multi-
chunk audit from a dead one (it now measures staleness from last_progress_at,
not started_at). Matches accumulate across chunks. soft/hard limits dropped
2h→15/16.7 min so the in-chunk budget fires first; a soft-limit backstop
re-enqueues to resume instead of erroring the whole run.

Tests: time-box → re-enqueue (status stays running); resume carries prior
matches and appends new ones. Existing full-scan tests unchanged (small sets
finish in one chunk).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

alembic/versions/0039_library_audit_resume.py

@@ -0,0 +1,40 @@
+"""library_audit_run: resume cursor + progress timestamp for chunked scans
+
+Revision ID: 0039
+Revises: 0038
+Create Date: 2026-06-07
+
+scan_library_for_rule used to run one 2h pass that timed out on large libraries
+and monopolized the concurrency-1 maintenance queue (operator-flagged). It now
+runs short time-boxed chunks that re-enqueue: `resume_after_id` persists the
+keyset cursor so the next chunk continues where it left off, and
+`last_progress_at` lets the recovery sweep tell a progressing multi-chunk audit
+from a genuinely stuck one.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0039"
+down_revision: Union[str, None] = "0038"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "library_audit_run",
+        sa.Column(
+            "resume_after_id", sa.Integer, nullable=False, server_default="0"
+        ),
+    )
+    op.add_column(
+        "library_audit_run",
+        sa.Column("last_progress_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("library_audit_run", "last_progress_at")
+    op.drop_column("library_audit_run", "resume_after_id")