fix: wire settings.json to actually drive runtime behavior

Settings page saved to /config/settings.json but nothing downstream read
that file. Schedule changes were silently ignored; max_quality and
sleep_interval changes were silently ignored. The "Settings saved
successfully" flash was a lie.

Fix:
- sync.sh reads max_quality + sleep_interval from settings.json on each
  run (jq -er ... // empty, falling back to env vars on missing/malformed
  file)
- entrypoint.sh reads sync_schedule from settings.json before setting up
  cron, and writes the crond PID to /var/run/crond.pid so Flask can
  SIGHUP it
- app.py adds apply_schedule(): rewrites /etc/crontabs/root, signals
  crond via the recorded PID, restarts crond if the PID is stale, drops
  the crontab when schedule is set to "manual". save_settings_route
  invokes it only when the schedule actually changed; any failure
  flashes a warning so the save still succeeds with the user informed
- bare `except: pass` in get_settings replaced with explicit exception
  types + stderr warning so debugging malformed settings is possible
- sync.sh: one bad channel no longer aborts the whole loop under set -e
- Dockerfile adds jq for the JSON reads in sync.sh / entrypoint.sh
- README: two stale github.com URLs fixed to Gitea; new Running Tests
  section under Building From Source
- tests/test_settings.py: 3 pytest cases covering get_settings()'s
  three branches (missing file, valid file, malformed JSON)

Settings hierarchy unchanged: env-var defaults seed the UI; settings.json
wins when present and parseable.

Timezone (TZ) is not applied live - tzdata is locked in at process start.
Same behavior as before; not in scope for this commit.

README.md

@@ -175,14 +175,24 @@ YouTube is blocking downloads. Solution: Add cookies.txt (see Configuration sect
 ## Building From Source
 
 ```bash
-git clone https://github.com/azcomputerguru/youtube-sync-docker
+git clone https://git.azcomputerguru.com/azcomputerguru/youtube-sync-docker
 cd youtube-sync-docker
 docker build -t youtube-sync .
 ```
 
+### Running Tests
+
+Tests are dev-only and not bundled into the production image. Install the
+dev dependencies and run pytest from the repo root:
+
+```bash
+pip install -r requirements-dev.txt
+pytest tests/
+```
+
 ## Support
 
-Issues and pull requests: https://github.com/azcomputerguru/youtube-sync-docker
+Issues and pull requests: https://git.azcomputerguru.com/azcomputerguru/youtube-sync-docker
 
 ## License