feat: Blinkist-style audio summary bot (MAT-74)

Add interactive article summary feature: user pastes URL → bot asks
language/duration/topics → generates audio summary via LLM + ElevenLabs
TTS → posts MP3 inline with transcript and follow-up Q&A.

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

article_summary/state.py

@@ -0,0 +1,60 @@
+"""Per-user FSM state machine for article summary conversations."""
+
+from __future__ import annotations
+
+import time
+from dataclasses import dataclass, field
+from enum import Enum, auto
+
+
+class ArticleState(Enum):
+    IDLE = auto()
+    URL_DETECTED = auto()
+    LANGUAGE = auto()
+    DURATION = auto()
+    TOPICS = auto()
+    GENERATING = auto()
+    COMPLETE = auto()
+
+
+@dataclass
+class ArticleSession:
+    state: ArticleState = ArticleState.IDLE
+    url: str = ""
+    title: str = ""
+    content: str = ""
+    language: str = ""
+    duration_minutes: int = 10
+    topics: list[str] = field(default_factory=list)
+    detected_topics: list[str] = field(default_factory=list)
+    summary_text: str = ""
+    timestamp: float = field(default_factory=time.time)
+
+
+STATE_TIMEOUT = 300  # 5 minutes
+
+
+class SessionManager:
+    """Manage per-(user, room) article summary sessions."""
+
+    def __init__(self) -> None:
+        self._sessions: dict[tuple[str, str], ArticleSession] = {}
+
+    def get(self, user_id: str, room_id: str) -> ArticleSession:
+        key = (user_id, room_id)
+        session = self._sessions.get(key)
+        if session and time.time() - session.timestamp > STATE_TIMEOUT:
+            session = None
+            self._sessions.pop(key, None)
+        if session is None:
+            session = ArticleSession()
+            self._sessions[key] = session
+        return session
+
+    def reset(self, user_id: str, room_id: str) -> None:
+        self._sessions.pop((user_id, room_id), None)
+
+    def touch(self, user_id: str, room_id: str) -> None:
+        key = (user_id, room_id)
+        if key in self._sessions:
+            self._sessions[key].timestamp = time.time()