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/components/shared/Sidebar.tsx

@@ -4,6 +4,7 @@ import Link from "next/link";
 import { usePathname, useRouter } from "next/navigation";
 import {
   Bookmark,
+  HelpCircle,
   LayoutDashboard,
   FileText,
   Users,
@@ -28,6 +29,7 @@ const NAV = [
   { href: "/following", label: "Following", icon: Heart, adminOnly: false, requiresAuth: true },
   { href: "/collections", label: "Collections", icon: Bookmark, adminOnly: false, requiresAuth: true },
   { href: "/notifications", label: "Notifications", icon: Bell, adminOnly: false, requiresAuth: true },
+  { href: "/how-it-works", label: "How it works", icon: HelpCircle, adminOnly: false, requiresAuth: false },
   { href: "/settings", label: "Admin", icon: Settings, adminOnly: true, requiresAuth: false },
 ];