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

frontend/lib/api.ts

@@ -85,6 +85,7 @@ export const billsAPI = {
     selected_points: string[];
     include_citations: boolean;
     zip_code?: string;
+    rep_name?: string;
   }) =>
     apiClient.post<{ draft: string }>(`/api/bills/${id}/draft-letter`, body).then((r) => r.data),
 };
@@ -132,6 +133,8 @@ export const membersAPI = {
     apiClient.get<PaginatedResponse<Member>>("/api/members", { params }).then((r) => r.data),
   get: (id: string) =>
     apiClient.get<Member>(`/api/members/${id}`).then((r) => r.data),
+  byZip: (zip: string) =>
+    apiClient.get<Member[]>(`/api/members/by-zip/${zip}`).then((r) => r.data),
   getBills: (id: string, params?: Record<string, unknown>) =>
     apiClient.get<PaginatedResponse<Bill>>(`/api/members/${id}/bills`, { params }).then((r) => r.data),
   getTrend: (id: string, days?: number) =>