feat(core): add Discord presence service and extract Jellyfin runtime composition

Introduce Discord presence runtime support and continue composition-root decomposition by moving Jellyfin wiring into dedicated composer modules. This keeps main runtime orchestration thinner while preserving behavior and test coverage across config, runtime, and docs updates.

docs/configuration.md

@@ -76,6 +76,7 @@ The configuration file includes several main sections:
 - [**Jimaku**](#jimaku) - Jimaku API configuration and defaults
 - [**AniList**](#anilist) - Optional post-watch progress updates
 - [**Jellyfin**](#jellyfin) - Optional Jellyfin auth, library listing, and playback launch
+- [**Discord Rich Presence**](#discord-rich-presence) - Optional Discord activity card updates
 - [**Keybindings**](#keybindings) - MPV command shortcuts
 - [**Runtime Option Palette**](#runtime-option-palette) - Live, session-only option toggles
 - [**Secondary Subtitles**](#secondary-subtitles) - Dual subtitle track support
@@ -479,8 +480,6 @@ Jellyfin integration is optional and disabled by default. When enabled, SubMiner
     "enabled": true,
     "serverUrl": "http://127.0.0.1:8096",
     "username": "",
-    "accessToken": "",
-    "userId": "",
     "remoteControlEnabled": true,
     "remoteControlAutoConnect": true,
     "autoAnnounce": false,
@@ -535,6 +534,57 @@ Launcher subcommand equivalents:
 
 Jellyfin remote auto-connect runs only when all three are `true`: `jellyfin.enabled`, `jellyfin.remoteControlEnabled`, and `jellyfin.remoteControlAutoConnect`.
 
+### Discord Rich Presence
+
+Discord Rich Presence is optional and disabled by default. When enabled, SubMiner publishes a polished activity card that reflects current media title, playback state, and session timer.
+
+```json
+{
+  "discordPresence": {
+    "enabled": true,
+    "clientId": "123456789012345678",
+    "detailsTemplate": "Watching {title}",
+    "stateTemplate": "{status}",
+    "largeImageKey": "subminer-logo",
+    "largeImageText": "SubMiner",
+    "smallImageKey": "study",
+    "smallImageText": "Sentence Mining",
+    "buttonLabel": "GitHub",
+    "buttonUrl": "https://github.com/sudacode/SubMiner",
+    "updateIntervalMs": 15000,
+    "debounceMs": 750
+  }
+}
+```
+
+| Option             | Values          | Description                                                                        |
+| ------------------ | --------------- | ---------------------------------------------------------------------------------- |
+| `enabled`          | `true`, `false` | Enable Discord Rich Presence updates (default: `false`)                            |
+| `clientId`         | string          | Discord application client ID                                                      |
+| `detailsTemplate`  | string          | Card details line template. Supports `{title}`, `{file}`, `{subtitle}`, `{status}` |
+| `stateTemplate`    | string          | Card state line template. Supports `{title}`, `{file}`, `{subtitle}`, `{status}`   |
+| `largeImageKey`    | string          | Large image asset key uploaded to your Discord app                                 |
+| `largeImageText`   | string          | Hover text for large image                                                         |
+| `smallImageKey`    | string          | Small image asset key uploaded to your Discord app                                 |
+| `smallImageText`   | string          | Hover text for small image                                                         |
+| `buttonLabel`      | string          | Optional button label (requires valid `buttonUrl`)                                 |
+| `buttonUrl`        | string (URL)    | Optional button URL shown on the activity card                                     |
+| `updateIntervalMs` | number          | Minimum interval between activity updates in milliseconds                          |
+| `debounceMs`       | number          | Debounce window for bursty playback events in milliseconds                         |
+
+Setup steps:
+
+1. Create a Discord application at <https://discord.com/developers/applications>.
+2. Open **Rich Presence > Art Assets** and upload image assets referenced by `largeImageKey` / `smallImageKey`.
+3. Copy the application ID into `discordPresence.clientId`.
+4. Set `discordPresence.enabled` to `true` and restart SubMiner.
+
+Troubleshooting:
+
+- If the card does not appear, verify Discord desktop app is running and `clientId` is correct.
+- If images do not render, confirm asset keys exactly match uploaded Discord asset names.
+- If Discord is closed/not installed/disconnects, SubMiner continues running and quietly skips presence updates.
+
 ### Keybindings
 
 Add a `keybindings` array to configure keyboard shortcuts that send commands to mpv: