Fix #218/#219/#220: identify ShadowBroker on Wikipedia + Wikidata calls (#284)

Wikimedia's User-Agent policy asks API clients to identify themselves
with a stable, contactable identifier so their operators can rate-limit
or coordinate. Before this change, ShadowBroker was sending:

- Backend (region_dossier.py): generic project default UA only; no
  Api-User-Agent.
- Frontend (useRegionDossier.ts, WikiImage.tsx, NewsFeed.tsx): zero
  identifying header at all; three separate copy-pasted anonymous
  fetches with their own module-local caches.

Three separate components doing the same broken thing meant policy
fixes had to happen in three places, with no shared cache or kill
switch.

Fix (no UX change, zero hostility):

== Backend ==

`backend/services/region_dossier.py` now sets explicit `User-Agent` +
`Api-User-Agent` headers on every outbound Wikidata and Wikipedia
request via a new `_WIKIMEDIA_REQUEST_HEADERS` constant. The identifier
includes a contact path (issues page on the public GitHub repo).

== Frontend ==

New shared helper `frontend/src/lib/wikimediaClient.ts`:
- `fetchWikipediaSummary(title)` — single source of truth for Wikipedia
  REST summary lookups, with one shared LRU cache (in-flight requests
  deduplicated, 512-entry cap), `Api-User-Agent` on every fetch.
- `fetchWikidataSparql(query)` — same shape for Wikidata SPARQL.
- `WIKIMEDIA_API_USER_AGENT` — exported constant; one place to update
  if Wikimedia ever asks us to back off.

Refactored three components to use the shared client:
- `frontend/src/hooks/useRegionDossier.ts` — fetchLeader() and
  fetchLocalWikiSummary() now route through the shared helpers.
- `frontend/src/components/WikiImage.tsx` — uses fetchWikipediaSummary,
  proper React state instead of module-mutation + forceUpdate trick.
- `frontend/src/components/NewsFeed.tsx` — same shape.

UX: byte-for-byte identical. Same thumbnails, same dossier content,
same load behavior. The only observable difference is the outgoing
request header.

Note on #239 (route duplication): an audit-grade inventory shows 166
main.py routes are shadowed by router modules. That cleanup is too
large to land safely in this PR; it will be staged as a separate
ladder of small PRs grouped by router module.

Tests:
- `backend/tests/test_region_dossier_wikimedia_ua.py` — 3 tests
  asserting backend headers are present.
- `frontend/src/__tests__/utils/wikimediaClient.test.ts` — 9 tests
  covering Api-User-Agent presence, shared cache, concurrent
  deduplication, disambiguation/HTTP-error/network-error fallthroughs,
  empty-input safety.

Local: backend 76/76 security suite green, frontend 716/716 vitest
suite green.

Credit: tg12 (external security audit).

backend/services/region_dossier.py

@@ -4,7 +4,7 @@ import concurrent.futures
 from urllib.parse import quote
 import requests as _requests
 from cachetools import TTLCache
-from services.network_utils import fetch_with_curl
+from services.network_utils import fetch_with_curl, DEFAULT_USER_AGENT
 
 logger = logging.getLogger(__name__)
 
@@ -15,6 +15,25 @@ dossier_cache = TTLCache(maxsize=500, ttl=86400)
 # Nominatim requires max 1 req/sec — track last call time
 _nominatim_last_call = 0.0
 
+# Issue #218 / #219 (tg12): Wikimedia's User-Agent policy requires API
+# clients to identify themselves with a stable User-Agent that includes
+# a contact path. Bare "python-requests/x.y" or generic strings violate
+# the policy and risk getting blocked. We send the project default UA
+# (operator-overridable via SHADOWBROKER_USER_AGENT) on EVERY outbound
+# Wikimedia request, plus the policy-recommended Api-User-Agent which
+# Wikimedia explicitly accepts on top of the regular UA.
+#
+# This is documented and stable so a Wikimedia operator who wants to
+# rate-limit or contact us has a fixed identifier to grep for.
+_WIKIMEDIA_REQUEST_HEADERS = {
+    "User-Agent": DEFAULT_USER_AGENT,
+    "Api-User-Agent": (
+        f"{DEFAULT_USER_AGENT} "
+        "(+https://github.com/BigBodyCobain/Shadowbroker; "
+        "report issues at /issues)"
+    ),
+}
+
 
 def _reverse_geocode_offline(lat: float, lng: float) -> dict:
     """Offline fallback via reverse_geocoder when external reverse geocoding is blocked."""
@@ -121,7 +140,13 @@ def _fetch_wikidata_leader(country_name: str) -> dict:
     """
     url = f"https://query.wikidata.org/sparql?query={quote(sparql)}&format=json"
     try:
-        res = fetch_with_curl(url, timeout=6)
+        # Issue #218 (tg12): Wikimedia's User-Agent policy requires
+        # outbound API traffic to be identifiable. fetch_with_curl()
+        # sends the project default, and we also add the Wikimedia-
+        # specific Api-User-Agent that the policy specifically asks
+        # for, since this request originates from a backend service
+        # that proxies on behalf of (potentially many) browser users.
+        res = fetch_with_curl(url, timeout=6, headers=_WIKIMEDIA_REQUEST_HEADERS)
         if res.status_code == 200:
             results = res.json().get("results", {}).get("bindings", [])
             if results:
@@ -147,7 +172,9 @@ def _fetch_local_wiki_summary(place_name: str, country_name: str = "") -> dict:
         slug = quote(name.replace(" ", "_"))
         url = f"https://en.wikipedia.org/api/rest_v1/page/summary/{slug}"
         try:
-            res = fetch_with_curl(url, timeout=5)
+            # Issue #219 (tg12): identify ourselves to Wikimedia per
+            # their UA policy; see _fetch_wikidata_leader above.
+            res = fetch_with_curl(url, timeout=5, headers=_WIKIMEDIA_REQUEST_HEADERS)
             if res.status_code == 200:
                 data = res.json()
                 if data.get("type") != "disambiguation":