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/api/bills.py

@@ -35,7 +35,8 @@ class DraftLetterRequest(BaseModel):
     tone: Literal["short", "polite", "firm"]
     selected_points: list[str]
     include_citations: bool = True
-    zip_code: str | None = None  # not stored, not logged
+    zip_code: str | None = None   # not stored, not logged
+    rep_name: str | None = None   # not stored, not logged
 
 
 class DraftLetterResponse(BaseModel):
@@ -50,6 +51,7 @@ async def list_bills(
     topic: Optional[str] = Query(None),
     sponsor_id: Optional[str] = Query(None),
     q: Optional[str] = Query(None),
+    has_document: Optional[bool] = Query(None),
     page: int = Query(1, ge=1),
     per_page: int = Query(20, ge=1, le=100),
     sort: str = Query("latest_action_date"),
@@ -80,6 +82,12 @@ async def list_bills(
                 Bill.short_title.ilike(f"%{q}%"),
             )
         )
+    if has_document is True:
+        doc_subq = select(BillDocument.bill_id).where(BillDocument.bill_id == Bill.bill_id).exists()
+        query = query.where(doc_subq)
+    elif has_document is False:
+        doc_subq = select(BillDocument.bill_id).where(BillDocument.bill_id == Bill.bill_id).exists()
+        query = query.where(~doc_subq)
 
     # Count total
     count_query = select(func.count()).select_from(query.subquery())
@@ -224,6 +232,7 @@ async def generate_letter(bill_id: str, body: DraftLetterRequest, db: AsyncSessi
             selected_points=body.selected_points,
             include_citations=body.include_citations,
             zip_code=body.zip_code,
+            rep_name=body.rep_name,
             llm_provider=llm_provider_override,
             llm_model=llm_model_override,
         )