fix(cctv): make the Houston TranStar pack opt-in

TranStar's disclaimer prohibits copying, distribution and external framing
of specific images without TxDOT's prior written permission, so the pack
no longer loads by default. It now uses envOptIn and loads only when
CCTV_HOUSTON_ENABLED=1, the same pattern as NYC DOT and Via Lietuva.

- DATA_SOURCES.md: replace the licence column with the disclaimer's terms
  and a link, and document the opt-in
- Ambient stills for Houston refresh every 3 minutes, matching TranStar's
  stated snapshot cadence
- README, CHANGELOG, CURRENT-STATE and .env.example describe the pack as
  opt-in; the README camera count no longer includes Houston
- Tests cover the default-off and =0 cases and the opted-in catalog path

src/data/cctvLod.js

@@ -34,6 +34,8 @@ const PROVIDER_STATIC_REFRESH_MS = Object.freeze({
   'ontario 511': 3 * 60 * 1000,
   // TxDOT publishes roughly once a minute; 3 min matches the other highway packs.
   txdot: 3 * 60 * 1000,
+  // TranStar says snapshots update about every 3 minutes.
+  'houston transtar': 3 * 60 * 1000,
   // Digitraffic publishes a new weathercam frame on each station's
   // collectionInterval, 600 s for every station sampled; polling faster only
   // re-fetches the same JPEG.