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

docs/CURRENT-STATE.md

@@ -1083,11 +1083,13 @@ URLs are built from each camera id under
 from the payload. Dedicated `dir` values become high-confidence headings;
 `None` uses the shared id-hash fallback at low confidence.
 
-By default the 250 cameras nearest downtown, Uptown, the Medical Center, IAH
-and Galveston load. `CCTV_HOUSTON_MAX_SOURCES` sets the pack cap (8–400) and
-`CCTV_HOUSTON_ENABLED=0` turns the pack off. The catalog-wide ceiling stays at
-4,000 and is shared round-robin, so adding this pack does not silently drop
-another region.
+TranStar's disclaimer prohibits redistribution and external framing of specific
+images without TxDOT's written permission, so the pack is off unless
+`CCTV_HOUSTON_ENABLED=1`. Opting in loads the 250 cameras nearest downtown,
+Uptown, the Medical Center, IAH and Galveston. `CCTV_HOUSTON_MAX_SOURCES` sets
+the pack cap (8–400). Snapshots update about every 3 minutes. The catalog-wide
+ceiling stays at 4,000 and is shared round-robin, so adding this pack does not
+silently drop another region.
 
 ## Location control ownership
 
@@ -3383,7 +3385,7 @@ its criteria cannot be silently ignored.
 | Satellites             | CelesTrak                                                                                                                                                                                       | `src/data/satellites.js`                              | `/api/celestrak`                                         | 120s                                                                              |
 | Space Missions (30d)   | Launch Library 2 + CelesTrak                                                                                                                                                                    | `src/data/rocketLaunches.js`                          | `/api/launches` + `/api/celestrak/active`                | 5 min                                                                             |
 | Traffic | Selectable TomTom / OpenStreetMap / Hybrid roads; optional TomTom flow (BYOK) | `src/data/traffic.js` | browser-direct OpenFreeMap tiles; `/api/tomtom` for flow | viewport-driven; capped tile cache |
-| CCTV                   | Austin + Caltrans (CA) + TfL London + Ontario 511 + Fintraffic (FI) + DriveBC (BC) + TxDOT (TX) + Estonia (Tallinn, Tarktee) + Live Traffic NSW + Open Calgary Open Data + Statens vegvesen (NO, stills + live HLS) + King County (WA) Open Data + WSDOT (WA) Open Data + NYC DOT TMC (opt-in) + City of Toronto RESCU + Catalonia (Servei Català de Trànsit) + Houston TranStar + Street View fallback | `src/data/cctv.js`                                    | `/api/cctv`                                              | 10s (active)                                                                      |
+| CCTV                   | Austin + Caltrans (CA) + TfL London + Ontario 511 + Fintraffic (FI) + DriveBC (BC) + TxDOT (TX) + Estonia (Tallinn, Tarktee) + Live Traffic NSW + Open Calgary Open Data + Statens vegvesen (NO, stills + live HLS) + King County (WA) Open Data + WSDOT (WA) Open Data + NYC DOT TMC (opt-in) + City of Toronto RESCU + Catalonia (Servei Català de Trànsit) + Houston TranStar (opt-in) + Street View fallback | `src/data/cctv.js`                                    | `/api/cctv`                                              | 10s (active)                                                                      |
 | Radio                  | Radio Browser (public-domain station directory)                                                                                                                                                 | `src/data/radio.js`                                   | `/api/radio/stations`, `/api/radio/click/:uuid`          | 45 min directory refresh                                                          |
 | Transit 🚌             | Operator GTFS-Realtime VehiclePositions (7 keyless regions, `src/data/transitFeeds.js`)                                                                                                         | `src/layers/transit/` via `src/app/layers/transit.js` | `/api/transit`                                           | 15s (poll + delayed playback)                                                     |
 | Bikeshare 🚲           | GBFS (Lyft + BCycle + Social Bicycles + PBSC incl. Bike Share Toronto, Bike Itaú (Brazil) and Bicing (Barcelona) + Urban Sharing, Entur, JCDecaux NO + nextbike (AMBici))                                                                                                                                        | `src/data/bikeshare.js`                               | `/api/gbfs`                                              | 60s                                                                               |