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/BillCard.tsx

@@ -1,5 +1,5 @@
 import Link from "next/link";
-import { TrendingUp, Calendar, User, FileText, FileClock, FileX } from "lucide-react";
+import { TrendingUp, Calendar, User, FileText, FileClock, FileX, Tag } from "lucide-react";
 import { Bill } from "@/lib/types";
 import { billLabel, chamberBadgeColor, cn, formatDate, partyBadgeColor, trendColor } from "@/lib/utils";
 import { FollowButton } from "./FollowButton";
@@ -28,12 +28,15 @@ export function BillCard({ bill, compact = false }: BillCardProps) {
               </span>
             )}
             {tags.map((tag) => (
-              <span
+              <Link
                 key={tag}
-                className="text-xs px-1.5 py-0.5 rounded-full bg-accent text-accent-foreground"
+                href={`/bills?topic=${tag}`}
+                onClick={(e) => e.stopPropagation()}
+                className="inline-flex items-center gap-0.5 text-xs px-1.5 py-0.5 rounded-full bg-accent text-accent-foreground hover:bg-accent/70 transition-colors"
               >
+                <Tag className="w-2.5 h-2.5" />
                 {tag}
-              </span>
+              </Link>
             ))}
           </div>