docs(cctv): say the Houston pack needs TxDOT's written permission to enable

TranStar's Content Usage terms prohibit copying, distribution and
external framing of its images without TxDOT's prior written
permission. The opt-in text only said the pack was off unless
CCTV_HOUSTON_ENABLED=1, without saying what an operator needs before
setting it. Say "enable only if you hold TxDOT's written permission" in
.env.example, the README, DATA_SOURCES.md, CURRENT-STATE and the
changelog, matching the NYC and Florida packs, and fix the column of the
.env.example comment.

The houston-cctv credit now reads "(Houston, when enabled)" and links
the Content Usage terms, like the Florida credit. Where the docs said
frames are "never stored or redistributed", they now say what happens:
frames are relayed on demand, buffered only per request, sent with
no-store and not archived.

changelog.d/1125-houston-cctv-pack.md

@@ -6,10 +6,13 @@
   snapshot id, and cameras outside Greater Houston, are dropped. TranStar's
   `dir` field (the side of the freeway the camera sits on) seeds a
   low-confidence heading along the nearest lanes' travel direction. TranStar's
-  disclaimer prohibits redistribution and external framing 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` changes that cap.
+  Content Usage terms (traffic.houstontranstar.org/disclaimer.aspx) prohibit
+  copying, distribution and external framing of its images without TxDOT's
+  written permission, so the pack is off by default; operators should set
+  `CCTV_HOUSTON_ENABLED=1` only if they hold that permission. Frames are
+  relayed on demand with `no-store` and not archived. Once enabled, it loads
+  the 250 cameras nearest downtown, Uptown, the Medical Center, IAH and
+  Galveston; `CCTV_HOUSTON_MAX_SOURCES` changes that cap.
   Snapshots update about every 3 minutes. The catalog-wide ceiling is
   unchanged; extra cameras share it round-robin with the other packs
   (mvanhorn, #1125).