ifta: recurring quarterly-return reminder campaign (calendar-triggered)

IFTA returns are due on fixed dates (Apr30/Jul31/Oct31/Jan31) and every
interstate carrier (op code A, ~628k sendable) files 4x/year forever -- pure
recurring revenue, no per-carrier deadline data needed.

- build_ifta_quarterly_campaign.py: self-gates to the reminder window (~21d
  before each deadline), selects interstate carriers, mints the same-day coupon,
  builds+schedules the campaign reusing build_trucking_campaigns plumbing (DRY:
  one source of truth for sending/suppression/coupon). Per-quarter cycle reset
  (ifta_reminder_cycle marker) so each quarter re-reminds the full pool; marks
  ifta_reminded_at to avoid double-sends within a cycle.
- ifta_quarterly_reminder.html: deadline + penalties + 'we do the math' + coupon
  + CAN-SPAM. Listmonk source campaign id 469.
- migration 094: fmcsa_carriers.ifta_reminded_at column + partial index.
Verified: deadline/window logic correct, imports reuse tc helpers, migration
applied on prod.

api/migrations/094_fmcsa_ifta_reminded.sql

@@ -0,0 +1,11 @@
+-- Track which interstate carriers have been sent the IFTA quarterly-return
+-- reminder this cycle, so the daily IFTA cron never double-sends within a quarter.
+-- The IFTA campaign builder resets this column at the start of each new quarter's
+-- reminder window (see build_ifta_quarterly_campaign.py).
+
+ALTER TABLE fmcsa_carriers
+  ADD COLUMN IF NOT EXISTS ifta_reminded_at TIMESTAMPTZ;
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_fmcsa_carriers_ifta_reminded
+  ON fmcsa_carriers (ifta_reminded_at)
+  WHERE ifta_reminded_at IS NULL;