[security] Close post-#227 control-surface and fetcher gaps

PR #227 hardened most Wormhole/Infonet control surfaces behind
require_local_operator and made the CrowdThreat fetcher opt-in. An
audit of the codebase against that PR's stated goals turned up four
classes of gap that the original change missed:

1. Two operator-only endpoints were left unprotected:
   - POST /api/wormhole/join: calls bootstrap_wormhole_identity() and
     flips the node into Tor mode, exactly the surface #227 hardened
     on /api/wormhole/identity/bootstrap.
   - POST /api/sigint/transmit: relays APRS-IS packets over radio
     using operator-supplied credentials. Anything that reached the
     API could transmit on the operator's authority.

   Both now require_local_operator. test_control_surface_auth.py
   extended with regression coverage for both.

2. Five third-party fetchers were still default-on, phoning home to
   politically/commercially sensitive upstreams on every poll cycle:
   - fimi.py            -> euvsdisinfo.eu        -> FIMI_ENABLED
   - prediction_markets -> Polymarket + Kalshi   -> PREDICTION_MARKETS_ENABLED
   - financial.py       -> Finnhub / yfinance    -> FINANCIAL_ENABLED or FINNHUB_API_KEY
   - nuforc_enrichment  -> huggingface.co        -> NUFORC_ENABLED
   - news.py            -> configured RSS feeds  -> NEWS_ENABLED (default on, kill switch)

   Same CrowdThreat-style pattern: explicit env-var opt-in, empty
   the data slot and mark_fresh when disabled. New regression test
   file test_third_party_fetchers_opt_in.py asserts each fetcher's
   network entry point is not called when its gate is off.

3. The outbound User-Agent leaked both the operator's personal email
   and a fork-specific GitHub URL on every fetcher request. Consolidated
   to a single DEFAULT_USER_AGENT in network_utils.py, project-generic
   by default (no contact info), overridable via SHADOWBROKER_USER_AGENT
   for operators who want to identify themselves (e.g. for Nominatim or
   weather.gov usage-policy compliance). Six call sites updated; the
   Nominatim-specific override is preserved.

4. The same generic UA now also flows through the peer prekey lookup
   in mesh_wormhole_prekey.py, so DM first-contact requests no longer
   identify the caller as a Shadowbroker fork to the peer being
   queried.

.env.example updated to document all new opt-in env vars.

Tests: backend/tests/test_control_surface_auth.py (extended),
       backend/tests/test_crowdthreat_opt_in.py (unchanged, still passes),
       backend/tests/test_third_party_fetchers_opt_in.py (new, 7 tests).
All 31 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/.env.example

@@ -24,8 +24,40 @@ AIS_API_KEY=              # https://aisstream.io/ — free tier WebSocket key
 # Requires MESH_DEBUG_MODE=true; do not enable this for ordinary use.
 # ALLOW_INSECURE_ADMIN=false
 
+# Default outbound User-Agent for all third-party HTTP fetchers.
+# Project-generic by default — does NOT include any personal contact info or
+# operator-specific identifier. Override only if you run a public relay and
+# want upstreams to be able to reach you (e.g. Nominatim/OSM usage policy).
+# SHADOWBROKER_USER_AGENT=ShadowBroker-OSINT/0.9 (contact: ops@example.com)
+
 # User-Agent for Nominatim geocoding requests (per OSM usage policy).
-# NOMINATIM_USER_AGENT=ShadowBroker/1.0 (https://github.com/BigBodyCobain/Shadowbroker)
+# NOMINATIM_USER_AGENT=ShadowBroker/1.0
+
+# ── Third-party fetcher opt-ins ────────────────────────────────
+# These data sources phone home to politically/commercially sensitive
+# upstreams. Disabled by default; set to "true" only if the operator
+# explicitly wants the node's IP to contact these services.
+#
+# CrowdThreat — backend.crowdthreat.world (paid threat-intel aggregator).
+# CROWDTHREAT_ENABLED=false
+#
+# EUvsDisinfo FIMI — euvsdisinfo.eu (EU disinformation tracker).
+# FIMI_ENABLED=false
+#
+# Polymarket + Kalshi — US political/election prediction markets.
+# PREDICTION_MARKETS_ENABLED=false
+#
+# Finnhub fallback / yfinance — financial market data.
+# Set FINNHUB_API_KEY to enable Finnhub, or set FINANCIAL_ENABLED=true to allow
+# the unauthenticated yfinance fallback to call Yahoo Finance.
+# FINANCIAL_ENABLED=false
+#
+# NUFORC UAP sightings — huggingface.co dataset download.
+# NUFORC_ENABLED=false
+#
+# News RSS aggregator — defaults ON. Set to "false" to disable all
+# configured news feeds (kill switch for the news layer).
+# NEWS_ENABLED=true
 
 # LTA Singapore traffic cameras — leave blank to skip this data source.
 # LTA_ACCOUNT_KEY=