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

.env.example

@@ -237,7 +237,7 @@ AISSTREAM_API_KEY=
 # CCTV_TORONTO_LIST_URL=                               # override the Toronto RESCU camera list URL
 # CCTV_LITHUANIA_ENABLED=                              # off by default; set 1 to load Via Lietuva / Eismoinfo road cameras (photo licence unconfirmed; see DATA_SOURCES.md)
 # CCTV_LITHUANIA_MAX_SOURCES=400                       # cap Lithuanian cameras; no API key required
-# CCTV_HOUSTON_ENABLED=1                               # set 0 to disable the Houston TranStar cameras
+# CCTV_HOUSTON_ENABLED=                                 # set to 1 to opt in; off by default (TranStar terms)
 # CCTV_HOUSTON_MAX_SOURCES=250                         # cap cameras loaded from the Houston TranStar catalog (nearest metro cores first)
 # CCTV_HOUSTON_CAMERAS_URL=                            # override the Houston TranStar catalog URL
 # CCTV_MAX_SOURCES=4000                                # catalog-wide ceiling, shared round-robin across packs