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.

src/data/dataCredits.js

@@ -321,8 +321,10 @@ export const DATA_CREDITS = [
   {
     key: 'houston-cctv',
     html:
-      'CCTV cameras & frames (Houston): ' +
-      '<a href="https://traffic.houstontranstar.org/cctv/transtar/" target="_blank" rel="noopener">Houston TranStar</a>',
+      'CCTV cameras &amp; frames (Houston, when enabled): ' +
+      '<a href="https://traffic.houstontranstar.org/cctv/transtar/" target="_blank" rel="noopener">Houston TranStar</a>. ' +
+      "Copying, distributing or framing its images needs TxDOT's written permission under the " +
+      '<a href="https://traffic.houstontranstar.org/disclaimer.aspx" target="_blank" rel="noopener">TranStar Content Usage terms</a>',
   },
   {
     key: 'hk-cctv',