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

app/models/sec_filing_gap.py

@@ -30,3 +30,8 @@ class SecFilingGap(Base):
     first_seen_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
     last_attempted_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
     escalated_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    # Set while this gap's issuer is exempt from the setup pause (escalated, and
+    # its own fundamentals still recent — see fundamentals_quality_service).
+    # Cleared when the exemption lapses, which is the moment the pause silently
+    # comes back and the only moment worth alerting on.
+    exempted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)