docs(readme): add Pixiv/DeviantArt to supported platforms, drop stale env var

- Supported platform list was missing Pixiv and DeviantArt even though both
  have gallery-dl extractors, PLATFORM_DEFAULTS entries, and extension
  platform definitions (Pixiv: OAuth token, DeviantArt: cookies).
- Drop DEFAULT_CHECK_INTERVAL from the env var table: no such env var
  exists. The default check interval is stored in the database under
  download.schedule_interval and edited via Settings → Download settings.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -23,7 +23,7 @@ The app handles rate limiting, retries, and tracks what's already been downloade
 - **Credential Management** - Securely encrypted storage for cookies/tokens
 - **Scheduled Downloads** - Automatic periodic checks for new content
 - **Real-time Updates** - WebSocket notifications for download progress
-- **Supported Platforms**: Patreon, SubscribeStar, Hentai Foundry, Discord
+- **Supported Platforms**: Patreon, SubscribeStar, Hentai Foundry, Discord, Pixiv, DeviantArt
 
 ## Quick Start
 
@@ -92,9 +92,12 @@ docker compose exec app alembic upgrade head
 | `PORT` | External port | `8080` |
 | `DOWNLOAD_PARALLEL_LIMIT` | Max concurrent downloads | `3` |
 | `DOWNLOAD_RATE_LIMIT` | Delay between requests (seconds) | `3.0` |
-| `DEFAULT_CHECK_INTERVAL` | Auto-check interval (seconds) | `28800` (8 hours) |
 | `LOG_LEVEL` | Logging verbosity | `INFO` |
 
+The default check interval (how often enabled sources are auto-checked) is
+stored in the database and editable in **Settings → Download settings**,
+not as an env var. Per-source overrides are also supported.
+
 See `.env.example` for the complete list with descriptions.
 
 ### Settings Page
@@ -117,6 +120,8 @@ The extension exports authentication cookies/tokens from supported platforms to
 | SubscribeStar | Cookies | Exports session cookies |
 | Hentai Foundry | Cookies | Exports session cookies |
 | Discord | Token | Automatically captures Authorization header |
+| Pixiv | OAuth token | Click to authenticate via Pixiv's OAuth flow |
+| DeviantArt | Cookies | Exports session cookies |
 
 ### Extension Setup