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>

hbd/server/notify.py

@@ -366,6 +366,9 @@ _TIMEOUT = 15  # seconds per channel send
 
 async def _dispatch_to_channel(channel_name: str, channel_cfg: dict, notif: Notification) -> bool:
     """Send *notif* to a single named channel, honouring min_level."""
+    # Strip ownership metadata — notifier drivers only need delivery credentials.
+    channel_cfg = {k: v for k, v in channel_cfg.items() if k not in ("owner", "private")}
+
     level = notif.level.upper()
     if level != "RECOVER":
         min_level = channel_cfg.get("min_level", "WARNING").upper()