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/services/llm_service.py

@@ -418,6 +418,7 @@ def generate_draft_letter(
     selected_points: list[str],
     include_citations: bool,
     zip_code: str | None,
+    rep_name: str | None = None,
     llm_provider: str | None = None,
     llm_model: str | None = None,
 ) -> str:
@@ -436,12 +437,19 @@ def generate_draft_letter(
 
     location_line = f"The constituent is writing from ZIP code {zip_code}." if zip_code else ""
 
+    if rep_name:
+        title = "Senator" if recipient == "senate" else "Representative"
+        salutation_instruction = f'- Open with "Dear {title} {rep_name},"'
+    else:
+        salutation_instruction = f'- Open with "Dear {chamber_word} Member,"'
+
     prompt = f"""Write a short constituent letter to a {chamber_word} member of Congress.
 
 RULES:
 - {tone_instruction}
 - 6 to 12 sentences total.
-- First sentence must be a clear, direct ask: "Please vote {vote_word} on {bill_label}."
+- {salutation_instruction}
+- Second sentence must be a clear, direct ask: "Please vote {vote_word} on {bill_label}."
 - The body must reference ONLY the points listed below — do not invent any other claims or facts.
 - {citation_instruction}
 - Close with a brief sign-off and the placeholder "[Your Name]".