feat: replace YAML notification channel editor with form-based UI

Notification channels are now managed through a proper web form instead
of a raw YAML textarea. Any authenticated user can create channels; private
channels (owner-scoped) are hidden from other users. The user profile
channel selector becomes a tag/chip picker with a "My Channels" CRUD section.

- settings.py: add CHANNEL_TYPE_SCHEMAS for all 6 notifier types; channel
  section switches to section_mode="channels"; cards include owner/private/min_level
- configio.py: add apply_channel() and delete_channel() for per-entry CRUD
- notify.py: strip owner/private metadata before dispatching to drivers
- http.py: add GET/POST /api/0/notification_channels, PUT/DELETE /{name},
  GET /api/0/notification_channel_types; visibility helper filters private
  channels per user; PUT /api/0/users/me validates against visible channels
- settings.html: card grid with edit/delete per channel; add/edit modal
  with type dropdown and dynamically rendered type-specific fields
- profile.html: chip picker replaces checkbox list; My Channels section
  for creating/editing/deleting user-owned channels
- tests: update test_settings_sections, test_http_users_me; add
  test_notification_channels_api (16 new tests, 46 total passing)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_settings_sections.py

@@ -24,7 +24,7 @@ def test_sections_have_section_mode():
     sections = settings_mod.get_settings_sections(CFG)
     for s in sections:
         assert "section_mode" in s, f"Section {s['id']} missing section_mode"
-        assert s["section_mode"] in ("form", "yaml")
+        assert s["section_mode"] in ("form", "yaml", "channels")
 
 
 def test_sections_have_api_section():
@@ -45,16 +45,41 @@ def test_network_section_has_editable_fields():
 def test_yaml_sections_have_correct_mode():
     sections = settings_mod.get_settings_sections(CFG)
     yaml_sections = {s["id"]: s for s in sections if s["section_mode"] == "yaml"}
-    assert "channels" in yaml_sections
+    assert "channels" not in yaml_sections  # now uses "channels" mode
     assert "hosts" in yaml_sections
     assert "thresholds" in yaml_sections
     assert "dns" in yaml_sections
-    assert yaml_sections["channels"]["api_section"] == "notification_channels"
     assert yaml_sections["hosts"]["api_section"] == "hosts"
     assert yaml_sections["thresholds"]["api_section"] == "thresholds"
     assert yaml_sections["dns"]["api_section"] == "dns"
 
 
+def test_channels_section_uses_channels_mode():
+    sections = settings_mod.get_settings_sections(CFG)
+    ch_sec = next(s for s in sections if s["id"] == "channels")
+    assert ch_sec["section_mode"] == "channels"
+    assert ch_sec["api_section"] == "notification_channels"
+    assert len(ch_sec["channels"]) == 1
+    ch = ch_sec["channels"][0]
+    assert ch["name"] == "pushover_ops"
+    assert ch["type"] == "pushover"
+    assert "owner" in ch
+    assert "private" in ch
+
+
+def test_channel_type_schemas_exported():
+    assert hasattr(settings_mod, "CHANNEL_TYPE_SCHEMAS")
+    for required_type in ("pushover", "email", "signal", "matrix", "sms_voipms"):
+        assert required_type in settings_mod.CHANNEL_TYPE_SCHEMAS
+        schema = settings_mod.CHANNEL_TYPE_SCHEMAS[required_type]
+        assert "label" in schema
+        assert "fields" in schema
+        for f in schema["fields"]:
+            assert "key" in f
+            assert "type" in f
+            assert "required" in f
+
+
 def test_oauth_section_exists():
     sections = settings_mod.get_settings_sections(CFG)
     oauth = next((s for s in sections if s["id"] == "oauth"), None)