fix(sec): alert when a filing gap's reprieve lapses instead of re-pausing quietly

An escalated gap stops pausing setups while the issuer's own fundamentals are
still recent. That reprieve ends on its own — the stored filings age past
GAP_GATE_RECENT_FILING_DAYS, or a newer gap arrives and the all-escalated
condition fails — and nothing reported either, because filing_gap_aged only
escalates gaps whose escalated_at is NULL and so never fires twice for the same
gap. For the 43 issuers behind the previous commit that lands around
2026-10-26, when their late-April filings age out together.

sec_filing_gaps.exempted_at (migration 034) makes the transition observable:
stamped quietly while the issuer is exempt, cleared when the exemption lapses,
and the clear is what raises filing_gap_repaused — once per lapse, re-arming if
the issuer's data recovers and ages out again. A gap that was never exempt has
no transition and stays silent; it is simply still paused, which
filing_gap_aged already said.

gap_exempt_ciks is public so the importer alerts on membership changes in
exactly the set the gate reads, rather than restating the rule.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Lo99z3jWqu9X9ueBU3Z3D

alembic/versions/034_sec_filing_gap_exemption.py

@@ -0,0 +1,41 @@
+"""Track when a filing gap stops pausing setups
+
+Revision ID: 034
+Revises: 033
+Create Date: 2026-08-21 00:00:00.000000
+
+An escalated gap stops pausing setups while the issuer's own fundamentals are
+still recent (``GAP_GATE_RECENT_FILING_DAYS``). That reprieve is not permanent:
+the stored filings age out, or a newer gap appears, and the pause returns —
+silently, because ``filing_gap_aged`` only escalates gaps whose ``escalated_at``
+is NULL and so never fires twice for the same gap.
+
+``exempted_at`` is the state marker that makes the transition observable. It is
+set (quietly) while the issuer is exempt and cleared when the exemption lapses,
+which is when ``filing_gap_repaused`` fires — once per lapse, re-arming if the
+issuer's data recovers and ages out again.
+
+Nullable, and carrying no meaning of its own beyond that state: an existing gap
+starts NULL and is stamped on the next import that finds it exempt.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = "034"
+down_revision: Union[str, None] = "033"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "sec_filing_gaps",
+        sa.Column("exempted_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("sec_filing_gaps", "exempted_at")