fix(plugins): remove the phantom http plugin; surface orphaned plugin settings

Operator saw the `http` plugin -- deleted when ping/dns/http were unified into
the Monitor entity -- still reported as enabled, with no way to clear it.

It was never an orphaned row. `plugin.http` was hardcoded in core DEFAULTS, so
deleting the app_settings row did nothing: the default reasserted it on the
next settings load. That is precisely why no cleanup UI could have fixed it.
Rule 22 says the removed subsystem should have taken its setting with it.

Purged the surviving references -- the DEFAULTS key, "http" in
CAPABILITY_PLUGINS, the stale plugin_manager docstring example, and the
capabilities blurb still advertising HTTP/uptime as a bundled capability --
plus a migration dropping any stored plugin.http row. Untouched: `http` as a
MONITOR TYPE (icmp/tcp/dns/http) everywhere it appears, and http_001_initial,
which is kept deliberately so existing DBs resolve the revision graph.

For the general case, cleanup splits by provenance rather than being uniformly
automatic or uniformly manual:

  Bundled plugins ship in the image and version atomically with core, so they
  cannot be transiently missing -- a plugin.* default with no bundled directory
  is unambiguously a bug. Guarded by a unit test that fails CI, which is the
  only part safe to automate.

  External plugins live in operator-mounted /data/plugins, where absence is
  ambiguous: unmounted volume, failed install, mid-upgrade. Auto-deleting their
  config would silently destroy unrecoverable credentials on a transient
  condition, so Settings now lists them under "Configured but not installed"
  with an explicit, confirmed, audit-logged Remove. The section hides entirely
  when empty, and the remove route refuses if the plugin is actually installed.

Orphan detection reads STORED rows, never the DEFAULTS-merged view -- offering
to remove a default-only key would be a lie -- and returns names only, since
plugin config can hold credentials and listing orphans never needs their values.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

steward/templates/settings/plugins.html

@@ -60,7 +60,7 @@
   </form>
 </div>
 <p style="color:var(--text-muted);font-size:0.82rem;margin:0 0 0.75rem;max-width:720px;">
-  Built-in ways to monitor your hosts (agent metrics, HTTP/uptime, SNMP, Docker). These are
+  Built-in ways to monitor your hosts (agent metrics, SNMP, Docker). These are
   facets of a host — you'll see their data in the <a href="/hosts/">Hosts</a> and
   <a href="/status">Status</a> sections, not as separate areas. On by default.
 </p>
@@ -88,6 +88,44 @@
 </div>
 {% endif %}
 
+{# ── Orphaned settings ─────────────────────────────────────────────────────── #}
+{# Hidden entirely when there is nothing to clean up — an empty "Configured but
+   not installed" panel would read as a problem rather than a clean state. #}
+{% if orphaned_plugins %}
+<div style="margin-bottom:1.5rem;max-width:720px;">
+  <div class="section-title" style="margin-bottom:0.4rem;">Configured but not installed</div>
+  <p style="color:var(--text-muted);font-size:0.82rem;margin:0 0 0.75rem;">
+    Stored settings for plugins Steward can't find. This usually means the plugin was
+    removed — but it also happens when an external plugin directory isn't mounted or an
+    install failed part-way, so nothing is deleted automatically. Removing an entry
+    discards that plugin's saved configuration, including any credentials.
+  </p>
+  <div style="display:grid;gap:0.6rem;">
+    {% for name in orphaned_plugins %}
+    <div class="card" style="padding:0.85rem 1rem;display:flex;align-items:center;gap:0.75rem;">
+      <span style="width:8px;height:8px;border-radius:50%;background:var(--yellow);flex-shrink:0;"
+            title="Configured but not installed"></span>
+      <div style="flex:1;min-width:0;">
+        <div style="display:flex;align-items:baseline;gap:0.5rem;flex-wrap:wrap;">
+          <span style="font-weight:600;font-size:0.9rem;color:var(--text);">{{ name }}</span>
+          <span style="font-size:0.72rem;padding:0.1em 0.45em;border-radius:3px;
+                       background:var(--yellow-dim);color:var(--yellow);">Not installed</span>
+        </div>
+        <div style="font-size:0.8rem;color:var(--text-muted);margin-top:0.1rem;">
+          Settings key <code>plugin.{{ name }}</code> has no matching plugin.
+        </div>
+      </div>
+      <form method="post" action="/settings/plugins/orphans/{{ name }}/remove/"
+            onsubmit="return confirm('Remove stored settings for &quot;{{ name }}&quot;? Its saved configuration, including any credentials, will be discarded. This cannot be undone.');"
+            style="margin:0;flex-shrink:0;">
+        <button type="submit" class="btn btn-danger btn-sm" style="font-size:0.78rem;">Remove</button>
+      </form>
+    </div>
+    {% endfor %}
+  </div>
+</div>
+{% endif %}
+
 {# ── Plugin Repositories ───────────────────────────────────────────────────── #}
 <div style="margin-bottom:2rem;max-width:720px;">
   <div class="section-title" style="margin-bottom:0.75rem;">Plugin Repositories</div>