fix(cctv): key-gate Ottawa, opt-in Québec video, keep shared slots

Review fixes for #1506.

- Ottawa: use the documented camera list and the city's Traffic Camera
  Images API. The pack is skipped with no request unless
  OTTAWA_CAMERA_CERTIFICATE is set; the certificate is added only at
  fetch time inside the pinned /opendata/ path, and each camera is
  fetched at most once a minute through the shared frame cache.
- Québec 511 video is opt-in (CCTV_QUEBEC_VIDEO_ENABLED=1) and loads
  only after the viewer presses Play; selection, auto-hop, scenes and
  voice no longer contact Québec 511. Credit splits locations (MTQ,
  CC BY 4.0) from video (Québec 511). Interrupted play() is no longer
  reported as an unavailable source; the per-frame listener runs only
  while a Québec camera is selected and the canvas rect is cached.
- Catalog cap: reserved Québec and World slots come only from capacity
  above the 4,000 shared slots, so a lower CCTV_MAX_SOURCES thins every
  pack instead of starving the existing ones. dev-fresh and
  .env.example default to 7,516.
- Throttled frames (World, Ottawa): inside the refresh floor with no
  cached frame the proxy serves the placeholder, never Street View; a
  failed fetch backs off for a minute; the cache is bounded by bytes.
- MJPEG first-frame extraction runs only for sources registered as
  MJPEG (Taiwan Highway Bureau), caps a frame at 2 MB, reads in linear
  time, accepts a CRLF preamble and a boundary that already starts with
  "--", and ends a part without Content-Length at the next delimiter.
  THB hosts are registered in the redirect pin list.
- Tests: World loader policy, Québec loader and opt-in, proxy cooldown
  without Street View, Ottawa signing, cap reservations at 7,516, 4,000
  and 48, overlay Play gate, refresh, visibility and teardown. Guard
  tests check the 4,000 shared slots again.
- Remove the unused standalone Québec preview module and public page.
- Docs: DATA_SOURCES, SECURITY, CURRENT-STATE, CCTV_INTEGRATION, README,
  .env.example, changelog fragment renamed to its full scope.

.env.example

@@ -211,6 +211,7 @@ AISSTREAM_API_KEY=
 # CCTV_TFL_MAX_SOURCES=250                             # cap cameras loaded from TfL
 # CCTV_ONTARIO_ENABLED=1                               # set 0 to disable Ontario 511 cameras
 # ONTARIO_511_API_KEY=                                 # free developer key: sign up at 511on.ca/my511/register, then request it at 511on.ca/developers/doc
+# OTTAWA_CAMERA_CERTIFICATE=                           # City of Ottawa Traffic Camera Images API certificate (free; request it via the form on traffic.ottawa.ca 'About the data'); without it the Ottawa pack is skipped
 # CCTV_ONTARIO_MAX_SOURCES=1000                        # cap cameras loaded from Ontario 511
 # CCTV_FINTRAFFIC_ENABLED=1                            # set 0 to drop the Fintraffic (Finland) pack
 # CCTV_FINTRAFFIC_MAX_SOURCES=300                      # cap cameras loaded from the Fintraffic catalog
@@ -258,7 +259,12 @@ AISSTREAM_API_KEY=
 # CCTV_TAIWAN_URL=                                     # override the Freeway Bureau MOTC 2.0 CCTV.xml URL
 # CCTV_A22_ENABLED=1                                   # set 0 to disable the Autostrada del Brennero (A22, Italy) webcams
 # CCTV_A22_URL=                                        # override the Open Data Hub A22 webcam catalog URL
-# CCTV_MAX_SOURCES=4000                                # catalog-wide ceiling, shared round-robin across packs
+# CCTV_OTTAWA_ENABLED=1                                # set 0 to skip City of Ottawa cameras (also needs OTTAWA_CAMERA_CERTIFICATE)
+# CCTV_OTTAWA_MAX_SOURCES=250                          # cap Ottawa cameras (8-500, nearest downtown first)
+# CCTV_QUEBEC_ENABLED=1                                # set 0 to skip the Québec camera locations (MTQ, Montréal, scenic links)
+# CCTV_QUEBEC_VIDEO_ENABLED=                           # off by default; set 1 to let viewers play Québec 511 clips in their browser after pressing Play (511 terms of use unconfirmed; see DATA_SOURCES.md)
+# CCTV_WORLD_CAMERAS_ENABLED=1                         # set 0 to skip the Iceland, Oregon and MLIT (Japan) still cameras
+# CCTV_MAX_SOURCES=7516                                # catalog-wide ceiling: 4,000 slots shared round-robin, the rest reserved for Québec and World
 # CCTV_PREFER_AUSTIN=1                                 # prioritize Austin cameras in the registry
 # CCTV_FORCE_AUSTIN=                                   # force live default sources when a source-pack JSON is configured