feat: ZIP → rep lookup, member page redesign, letter improvements

ZIP lookup (GET /api/members/by-zip/{zip}):
- Two-step geocoding: Nominatim (ZIP → lat/lng) then Census TIGERweb
  Legislative identify (lat/lng → congressional district via GEOID)
- Handles at-large states (AK, DE, MT, ND, SD, VT, WY)
- Added rep_lookup health check to admin External API Health panel

congress_api.py fixes:
- parse_member_from_api: normalize state full name → 2-letter code
  (Congress.gov returns "Florida", DB expects "FL")
- parse_member_from_api: read district from top-level data field,
  not current_term (district is not inside the term object)

Celery beat: schedule sync_members daily at 1 AM UTC so chamber,
district, and contact info stay current without manual triggering

Members page redesign: photo avatars, party/state/chamber chips,
phone + website links, ZIP lookup form to find your reps

Draft letter improvements: pass rep_name from ZIP lookup so letter
opens with "Dear Representative Franklin," instead of generic salutation;
add has_document filter to bills list endpoint

UX additions: HelpTip component, How It Works page, "How it works"
sidebar nav link, collections page description copy

Authored-By: Jack Levy

backend/app/workers/celery_app.py

@@ -70,6 +70,10 @@ celery_app.conf.update(
             "task": "app.workers.member_interest.calculate_all_member_trend_scores",
             "schedule": crontab(hour=3, minute=0),
         },
+        "sync-members": {
+            "task": "app.workers.congress_poller.sync_members",
+            "schedule": crontab(hour=1, minute=0),  # 1 AM UTC daily — refreshes chamber/district/contact info
+        },
         "fetch-actions-active-bills": {
             "task": "app.workers.congress_poller.fetch_actions_for_active_bills",
             "schedule": crontab(hour=4, minute=0),  # 4 AM UTC, after trend + member scoring