docs: update notification channel and API docs for form-based management

- NOTIFICATIONS.md: document owner/private fields, channel visibility
  rules, and user-created channels; add troubleshooting note for
  private channel visibility
- HTTP_API.md: add notification channel API endpoints table and full
  endpoint reference (GET types, GET/POST/PUT/DELETE channels)
- USERS.md: add missing PUT /api/0/users/me endpoint documentation
  with all three update modes (identity, channels, password)

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

docs/HTTP_API.md

@@ -53,6 +53,17 @@ See [User Management](USERS.md) for full authentication documentation.
 |--------|------|-------------|------|
 | `GET` | `/api/0/users` | List all users | Admin |
 | `GET` | `/api/0/users/me` | Own profile | Authenticated |
+| `PUT` | `/api/0/users/me` | Update own profile | Authenticated |
+
+### Notification Channels
+
+| Method | Path | Description | Role |
+|--------|------|-------------|------|
+| `GET` | `/api/0/notification_channel_types` | Channel type schemas | Authenticated |
+| `GET` | `/api/0/notification_channels` | List visible channels | Authenticated |
+| `POST` | `/api/0/notification_channels` | Create a channel | Authenticated |
+| `PUT` | `/api/0/notification_channels/{name}` | Update a channel | Owner or Admin |
+| `DELETE` | `/api/0/notification_channels/{name}` | Delete a channel | Owner or Admin |
 
 ### Host Management
 
@@ -203,6 +214,101 @@ Changes take effect immediately but are not written back to the config file. Upd
 
 ---
 
+---
+
+### Notification Channel Endpoints
+
+Channels are visible to all users by default. Channels marked `private: true` are only visible to their owner. Admins see all channels.
+
+#### GET /api/0/notification_channel_types
+Return the schema for every supported notifier type. Used by the web UI to dynamically render the channel creation form.
+
+**Response:**
+```json
+{
+  "pushover": {
+    "label": "Pushover",
+    "fields": [
+      {"key": "token",  "label": "App token",  "type": "secret", "required": true},
+      {"key": "user",   "label": "User key",   "type": "secret", "required": true},
+      {"key": "sound",  "label": "Sound",      "type": "text",   "required": false}
+    ]
+  },
+  "email": { "label": "E-mail", "fields": [ ... ] },
+  ...
+}
+```
+
+---
+
+#### GET /api/0/notification_channels
+List channels visible to the current user (public channels + own private channels). Admins receive all channels.
+
+**Response:**
+```json
+[
+  {
+    "name": "pushover_ops",
+    "type": "pushover",
+    "type_label": "Pushover",
+    "owner": null,
+    "private": false,
+    "min_level": "WARNING",
+    "fields": [
+      {"key": "token", "label": "App token", "value": "•••", "sensitive": true},
+      {"key": "user",  "label": "User key",  "value": "•••", "sensitive": true}
+    ]
+  }
+]
+```
+
+Sensitive fields (`type: "secret"`) are always returned as `"•••"`.
+
+---
+
+#### POST /api/0/notification_channels
+Create a new channel. The creating user becomes the channel's `owner`.
+
+**Request body:**
+```json
+{
+  "name": "my_pushover",
+  "type": "pushover",
+  "token": "app-token",
+  "user": "user-key",
+  "min_level": "WARNING",
+  "private": true
+}
+```
+
+**Response:** `{"ok": true, "name": "my_pushover"}`
+
+**Status codes:** `200 OK`, `400` (missing required field or unknown type), `409` (name already exists)
+
+---
+
+#### PUT /api/0/notification_channels/{name}
+Update an existing channel. Only the channel owner or an admin may update it.
+
+Secret fields sent as `"•••"` are preserved from the existing config (same pattern as OAuth secrets in the admin config editor).
+
+**Request body:** same shape as POST, `name` ignored (taken from URL).
+
+**Response:** `{"ok": true}`
+
+**Status codes:** `200 OK`, `403 Forbidden`, `404 Not Found`
+
+---
+
+#### DELETE /api/0/notification_channels/{name}
+Delete a channel. Only the channel owner or an admin may delete it.
+
+**Response:** `{"ok": true}`
+
+**Status codes:** `200 OK`, `403 Forbidden`, `404 Not Found`
+
+---
+
 ### Alert Endpoints
 
 #### GET /api/0/hosts/{hostname}/alerts