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/app/collections/page.tsx

@@ -5,6 +5,7 @@ import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import Link from "next/link";
 import { Bookmark, Plus, Globe, Lock, Trash2 } from "lucide-react";
 import { collectionsAPI } from "@/lib/api";
+import { HelpTip } from "@/components/shared/HelpTip";
 import type { Collection } from "@/lib/types";
 
 export default function CollectionsPage() {
@@ -49,9 +50,15 @@ export default function CollectionsPage() {
   return (
     <div className="max-w-2xl mx-auto space-y-6">
       <div className="flex items-center justify-between">
-        <div className="flex items-center gap-2">
-          <Bookmark className="w-5 h-5 text-primary" />
-          <h1 className="text-xl font-bold">My Collections</h1>
+        <div>
+          <div className="flex items-center gap-2">
+            <Bookmark className="w-5 h-5 text-primary" />
+            <h1 className="text-xl font-bold">My Collections</h1>
+          </div>
+          <p className="text-sm text-muted-foreground mt-1">
+            A collection is a named group of bills you curate — like a playlist for legislation.
+            Share any collection via a link; no account required to view.
+          </p>
         </div>
         <button
           onClick={() => setShowForm((v) => !v)}
@@ -87,7 +94,7 @@ export default function CollectionsPage() {
               className="rounded"
             />
             <span>Public collection</span>
-            <span className="text-xs text-muted-foreground">(share link works either way)</span>
+            <HelpTip content="Share links work whether or not a collection is public. Marking it public signals it may appear in a future public directory — private collections are invisible to anyone without your link." />
           </label>
           {formError && <p className="text-xs text-destructive">{formError}</p>}
           <div className="flex gap-2">