feat(platform): tini as PID 1, Hypercorn, a log-level setting, the image on CI's Python, an airgapped boot gate before push (#5159)

- Dockerfile: python:3.14-slim, the Python CI tests on (was 3.13). tini from
  apt is PID 1 (ENTRYPOINT tini -- entrypoint.sh): it reaps the zombies ssh
  and ansible leave and forwards SIGTERM.
- cli: Hypercorn's serve() with one worker (run, stream and scheduler state is
  in-process), a 10 s graceful timeout, and SIGTERM/SIGINT as the shutdown
  trigger. Access and error lines go through Steward's logging. --debug keeps
  Quart's reloading dev server.
- Logging: steward/core/logging_setup.py installs one root handler before
  anything logs (until now a library's basicConfig on import was the only
  configuration). Its level is the `log.level` setting (DEBUG/INFO/WARNING/
  ERROR, default INFO), applied at boot and on save in Settings → General.
- CI publish: build, then "The image boots with no internet" (rule 164,
  snippet #3552), then push, so a broken image never reaches a tag (rule 177).
  On an --internal network with its own Postgres, the image must migrate and
  answer /health, run tini as PID 1, serve first-run setup with its assets
  from /static and no third-party host, and exit 0 on SIGTERM. Logs and
  clean-up run always.

Tests: tests/core/test_logging_setup.py; the smoke step itself.
Not in this commit: the dependency lockfile (it conflicts with rule 154 and
is waiting on the operator).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/settings/general.html

@@ -74,6 +74,16 @@
            value="{{ settings['monitors.poll_interval_seconds'] }}" style="max-width:120px;">
     <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.25rem;">Requires restart to take effect.</div>
   </div>
+
+  <div class="form-group">
+    <label>Log level</label>
+    <select name="log.level" style="max-width:160px;">
+      {% for level in log_levels %}
+      <option value="{{ level }}" {% if settings['log.level'] == level %}selected{% endif %}>{{ level | capitalize }}</option>
+      {% endfor %}
+    </select>
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.25rem;">What reaches the container log. Applies when saved. Warning hides the line logged for each request.</div>
+  </div>
 </div>
 
 <div style="margin-top:1rem;display:flex;align-items:center;gap:1rem;">