feat(docker): container log viewer + Settings controls [M79 step 5]

The user-facing half (rule 27). Per-container log viewer + admin controls, so
container logs are something the operator can actually touch.

Viewer (plugins/docker):
- /container/<host>/<name>/logs full page + /logs/lines HTMX fragment polled
  every 5s (near-live follow); stream filter (all/stdout/stderr) + case-insensitive
  text search; newest-first so the latest lines survive the poll's scroll reset;
  empty/loading states; a "View logs" link from the container detail page.
  Jinja auto-escapes log content (no markup injection).

Settings (Thresholds & Retention tab):
- global on/off toggle, comma-separated exclude list, retention days + max MB
  per container — DB-backed, no restart. The toggle + exclude are enforced
  authoritatively at ingest (_persist_logs drops disabled/excluded lines), since
  the push model has no channel to tell an agent to stop.

Tests: route-defined smoke; template-parse covers the new templates; integration
test for the ingest-time toggle + exclude enforcement.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CAGR73DUowdVFVvYzLXC5C

steward/templates/settings/thresholds.html

@@ -70,6 +70,54 @@
           "Keep container start/stop/die/health history this long.") }}
 </div>
 
+<div class="card" style="max-width:640px;margin-top:1rem;">
+  <h2 class="section-title" style="margin-bottom:0.5rem;">Container logs</h2>
+  <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">
+    The host agent tails each container's logs and pushes them here, viewable per
+    container. On by default for every container; storage is bounded by a
+    per-container ring (oldest lines rotate out once the age or size cap is hit,
+    whichever comes first). Applied by the hourly cleanup and on ingest.
+  </p>
+
+  <div class="form-group" style="margin-bottom:1.1rem;">
+    <label style="display:flex;align-items:center;gap:0.5rem;cursor:pointer;">
+      <input type="checkbox" name="docker_logs_enabled" style="width:auto;"
+             {% if settings["docker.logs.enabled"] %}checked{% endif %}>
+      Collect container logs
+    </label>
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">
+      Global kill-switch. When off, pushed log lines are dropped and no new logs are stored.
+    </div>
+  </div>
+
+  <div class="form-group" style="margin-bottom:1.1rem;">
+    <label>Exclude containers</label>
+    <div style="margin-top:0.25rem;">
+      <input type="text" name="docker_logs_exclude"
+             value="{{ settings['docker.logs.exclude'] | join(', ') }}"
+             placeholder="watchtower, some-chatty-service" style="width:100%;max-width:420px;">
+    </div>
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">
+      Comma-separated container names whose logs are dropped on ingest (e.g. known-noisy ones).
+    </div>
+  </div>
+
+  {{ days("Log retention", "docker_logs_retention_days", "docker.logs.retention_days",
+          "Keep each container's log lines at most this long before rotating them out.") }}
+
+  <div class="form-group" style="margin-bottom:0.25rem;">
+    <label>Max size per container <span style="color:var(--text-muted);font-size:0.8rem;">(MB)</span></label>
+    <div style="margin-top:0.25rem;">
+      <input type="number" name="docker_logs_max_mb" min="1" step="1"
+             value="{{ (settings['docker.logs.max_bytes_per_container'] // 1000000) or 1 }}"
+             style="max-width:110px;">
+    </div>
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">
+      Newest lines are kept up to this size per container; older lines rotate out first.
+    </div>
+  </div>
+</div>
+
 <div class="card" style="max-width:640px;margin-top:1rem;">
   <h2 class="section-title" style="margin-bottom:0.5rem;">Host metrics retention</h2>
   <p style="font-size:0.82rem;color:var(--text-muted);margin-bottom:1.25rem;">