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>

tests/test_twitch.py

@@ -17,6 +17,7 @@ sys.modules[_spec.name] = _mod
 _spec.loader.exec_module(_mod)
 
 from plugins.twitch import (  # noqa: E402
+    _compact_num,
     _delete,
     _errors,
     _load,
@@ -652,6 +653,17 @@ class TestCmdTwitchList:
         assert "broken (error)" in bot.replied[0]
 
     def test_list_shows_live(self):
+        _clear()
+        bot = _FakeBot()
+        _save(bot, "#test:xqc", {
+            "name": "xqc", "channel": "#test",
+            "last_error": "", "was_live": True,
+            "last_viewers": 50000,
+        })
+        asyncio.run(cmd_twitch(bot, _msg("!twitch list")))
+        assert "xqc (live, 50k)" in bot.replied[0]
+
+    def test_list_shows_live_no_viewers(self):
         _clear()
         bot = _FakeBot()
         _save(bot, "#test:xqc", {
@@ -725,8 +737,10 @@ class TestCmdTwitchCheck:
             assert len(announcements) == 1
             assert "[xqc] is live" in announcements[0]
             assert "Fortnite" in announcements[0]
-            # Check reply shows live status
+            assert "| 50k viewers" in announcements[0]
+            # Check reply shows live status with viewers
             assert "xqc: live" in bot.replied[0]
+            assert "| 50k viewers" in bot.replied[0]
 
         asyncio.run(inner())
 
@@ -794,6 +808,7 @@ class TestPollOnce:
             assert "[xqc] is live" in messages[0]
             assert "Playing games" in messages[0]
             assert "Fortnite" in messages[0]
+            assert "| 50k viewers" in messages[0]
             assert "https://twitch.tv/xqc" in messages[0]
             updated = _load(bot, key)
             assert updated["was_live"] is True
@@ -941,6 +956,7 @@ class TestPollOnce:
             assert len(messages) == 1
             assert "Just chatting" in messages[0]
             assert "(" not in messages[0]  # No game parenthetical
+            assert "| 100 viewers" in messages[0]
 
         asyncio.run(inner())
 
@@ -1132,3 +1148,33 @@ class TestCmdTwitchUsage:
         bot = _FakeBot()
         asyncio.run(cmd_twitch(bot, _msg("!twitch foobar")))
         assert "Usage:" in bot.replied[0]
+
+
+# ---------------------------------------------------------------------------
+# TestCompactNum
+# ---------------------------------------------------------------------------
+
+class TestCompactNum:
+    def test_zero(self):
+        assert _compact_num(0) == "0"
+
+    def test_small(self):
+        assert _compact_num(999) == "999"
+
+    def test_one_k(self):
+        assert _compact_num(1000) == "1k"
+
+    def test_fractional_k(self):
+        assert _compact_num(1500) == "1.5k"
+
+    def test_one_m(self):
+        assert _compact_num(1_000_000) == "1M"
+
+    def test_fractional_m(self):
+        assert _compact_num(2_500_000) == "2.5M"
+
+    def test_fifty_k(self):
+        assert _compact_num(50000) == "50k"
+
+    def test_hundred(self):
+        assert _compact_num(100) == "100"