feat: metadata enrichment for alerts and subscription plugins

Alert backends now populate structured `extra` field with engagement
metrics (views, stars, votes, etc.) instead of embedding them in titles.
Subscription plugins show richer announcements: Twitch viewer counts,
YouTube views/likes/dates, RSS published dates.

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

docs/USAGE.md

@@ -572,7 +572,8 @@ Polling and announcements:
 
 - Feeds are polled every 10 minutes by default
 - On `add`, existing items are recorded without announcing (prevents flood)
-- New items are announced as `[name] title -- link`
+- New items are announced as `[name] title | YYYY-MM-DD -- link`
+- Published date is included when available (RSS `pubDate`, Atom `published`)
 - Maximum 5 items announced per poll; excess shown as `... and N more`
 - Titles are truncated to 80 characters
 - Supports HTTP conditional requests (`ETag`, `If-Modified-Since`)
@@ -608,7 +609,8 @@ Polling and announcements:
 
 - Channels are polled every 10 minutes by default
 - On `follow`, existing videos are recorded without announcing (prevents flood)
-- New videos are announced as `[name] Video Title -- https://www.youtube.com/watch?v=ID`
+- New videos are announced as `[name] Video Title | 1.5Mv 45klk 2026-01-15 -- URL`
+- Metadata suffix includes views, likes, and published date when available
 - Maximum 5 videos announced per poll; excess shown as `... and N more`
 - Titles are truncated to 80 characters
 - Supports HTTP conditional requests (`ETag`, `If-Modified-Since`)
@@ -639,11 +641,12 @@ Polling and announcements:
 - Streamers are polled every 2 minutes by default
 - On `follow`, the current stream state is recorded without announcing
 - Announcements fire on state transitions: offline to live, or new stream ID
-- Format: `[name] is live: Stream Title (Game) -- https://twitch.tv/login`
-- Game is omitted if not set; titles are truncated to 80 characters
+- Format: `[name] is live: Stream Title (Game) | 50k viewers -- https://twitch.tv/login`
+- Game is omitted if not set; viewer count shown when available
+- Titles are truncated to 80 characters
 - 5 consecutive errors doubles the poll interval (max 1 hour)
 - Subscriptions persist across bot restarts via `bot.state`
-- `list` shows live/error status indicators next to each streamer
+- `list` shows live/error status with viewer count: `name (live, 50k)`
 - `check` forces an immediate poll and reports current status
 
 ### `!searx` -- SearXNG Web Search
@@ -720,6 +723,26 @@ Platforms searched:
 - **Medium** (`md`) -- Tag-based RSS feed (no auth required)
 - **Hugging Face** (`hf`) -- Model search API, sorted by downloads (no auth required)
 
+Backend metadata (shown as `| extra` suffix on titles):
+
+| Tag | Metrics | Example |
+|-----|---------|---------|
+| `tw` | viewers / views | `500 viewers`, `1k views` |
+| `rd` | score, comments | `+127 42c` |
+| `ft` | reblogs, favourites | `3rb 12fav` |
+| `bs` | likes, reposts | `5lk 2rp` |
+| `ly` | score, comments | `+15 3c` |
+| `kk` | viewers | `500 viewers` |
+| `dm` | views | `1.2k views` |
+| `pt` | views, likes | `120v 5lk` |
+| `hn` | points, comments | `127pt 42c` |
+| `gh` | stars, forks | `42* 5fk` |
+| `gl` | stars, forks | `42* 5fk` |
+| `se` | score, answers, views | `+5 3a 1.2kv` |
+| `dh` | stars, pulls | `42* 1.2M pulls` |
+| `hf` | downloads, likes | `500dl 12lk` |
+| `dv` | reactions, comments | `+15 3c` |
+
 Polling and announcements:
 
 - Alerts are polled every 5 minutes by default
@@ -727,7 +750,7 @@ Polling and announcements:
   background to avoid flooding
 - New results announced as two lines:
   - ACTION: `* derp [name/<tag>/<id>] date - URL`
-  - PRIVMSG: full uncropped title/content
+  - PRIVMSG: `title | extra` (title with compact engagement metrics when available)
 - Tags: `yt`, `tw`, `sx`, `rd`, `ft`, `dg`, `gn`, `kk`, `dm`, `pt`, `bs`, `ly`,
   `od`, `ia`, `hn`, `gh`, `wp`, `se`, `gl`, `nm`, `pp`, `dh`, `ax`, `lb`, `dv`,
   `md`, `hf` -- `<id>` is a short deterministic ID for use with `!alert info`