feat(ansible): runner robustness — cancel, concurrency, structured results, retention

Closes #550 (all four):

- Cancellation: track live subprocesses; POST /ansible/runs/<id>/cancel
  (operator) SIGTERMs then SIGKILLs after a grace; new 'cancelled' status
  (+ migration 0019, ALTER TYPE in autocommit). Queued runs cancel cleanly
  before launch. Cancel button on run detail.
- Concurrency: global semaphore (ansible.max_concurrent_runs, default 3,
  Settings→Ansible) caps simultaneous runs; excess show 'queued' (new status)
  until a slot frees. Semaphore bound lazily per running loop.
- Structured results: parse PLAY RECAP into per-host ok/changed/unreachable/
  failed/skipped + capture failed-task lines, stored in new results JSON
  column (migration 0020); rendered as a host-summary table on run detail.
  Keeps live streaming (no json-callback swap).
- Retention: full output written to a persistent log artifact
  (/data/ansible/runs/<id>.log, env-overridable) beyond the 1 MB DB cap and
  across restarts; in-memory replay buffer bounded + GC'd after completion;
  Download-log route. Boot reconciliation now also sweeps stale 'queued'.

Unit tests for recap parsing + cancel flagging. Status colors updated across
run list / detail / schedules.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/templates/settings/ansible.html

@@ -62,6 +62,13 @@
           Enforce SSH host-key checking <span style="color:var(--text-muted);font-size:0.78rem;">(off by default for homelab convenience)</span>
         </label>
       </div>
+      <div class="form-group">
+        <label>Max concurrent runs
+          <span style="color:var(--text-muted);font-size:0.78rem;font-weight:normal;">(extra runs queue; applied on restart)</span>
+        </label>
+        <input type="number" name="max_concurrent_runs" min="1" max="50"
+               value="{{ max_concurrent_runs }}" style="width:7rem;">
+      </div>
       <button type="submit" class="btn">Save credentials</button>
     </form>
   </div>