feat(messages): add Redis service for pub/sub and presence

Singleton Redis client module with graceful fallback when Redis is
unavailable. Provides pub/sub (SSE events) and presence tracking
(online status, last_seen) for the messaging redesign.

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

app.py

@@ -262,6 +262,9 @@ else:
     limiter._storage_uri = "memory://"
 limiter.init_app(app)
 
+from redis_service import init_redis
+init_redis(app)
+
 @limiter.request_filter
 def is_admin_exempt():
     """Exempt logged-in admins from rate limiting."""

redis_service.py

@@ -0,0 +1,141 @@
+"""
+Redis service for pub/sub (SSE events) and presence tracking.
+Uses same Redis instance as Flask-Limiter (localhost:6379/0).
+"""
+
+import json
+import logging
+from datetime import datetime, timezone
+
+logger = logging.getLogger(__name__)
+
+_redis_client = None
+_redis_available = False
+
+
+def init_redis(app=None):
+    """Connect to Redis, set global _redis_client and _redis_available.
+    Graceful fallback if Redis unavailable.
+    """
+    global _redis_client, _redis_available
+    try:
+        import redis
+        client = redis.Redis(host='localhost', port=6379, db=0,
+                             decode_responses=True,
+                             socket_connect_timeout=2,
+                             socket_timeout=2)
+        client.ping()
+        _redis_client = client
+        _redis_available = True
+        logger.info("Redis service initialized (localhost:6379/0)")
+    except Exception as e:
+        _redis_client = None
+        _redis_available = False
+        logger.warning(f"Redis service unavailable, pub/sub and presence disabled: {e}")
+
+
+def get_redis():
+    """Return Redis client or None if unavailable."""
+    return _redis_client
+
+
+def is_available():
+    """Return True if Redis is available."""
+    return _redis_available
+
+
+# ---------------------------------------------------------------------------
+# Pub/Sub helpers
+# ---------------------------------------------------------------------------
+
+def publish_event(user_id, event_type, data):
+    """Publish a JSON event to user:{user_id}:events channel.
+
+    Args:
+        user_id: Target user ID.
+        event_type: String event type (e.g. 'new_message', 'typing', 'reaction').
+        data: Dict payload to include in the event.
+    """
+    if not _redis_available or _redis_client is None:
+        return
+    try:
+        channel = f"user:{user_id}:events"
+        payload = json.dumps({"type": event_type, **data})
+        _redis_client.publish(channel, payload)
+    except Exception as e:
+        logger.error(f"publish_event failed for user {user_id}: {e}")
+
+
+def publish_to_conversation(db, conversation_id, event_type, data, exclude_user_id=None):
+    """Publish an event to all members of a conversation.
+
+    Queries ConversationMember for all members of the conversation and
+    calls publish_event for each (except excluded user).
+
+    Args:
+        db: SQLAlchemy db instance.
+        conversation_id: ID of the conversation.
+        event_type: String event type.
+        data: Dict payload.
+        exclude_user_id: Optional user ID to skip (e.g. the sender).
+    """
+    if not _redis_available or _redis_client is None:
+        return
+    try:
+        from database import ConversationMember
+        members = db.session.query(ConversationMember).filter_by(
+            conversation_id=conversation_id
+        ).all()
+        for member in members:
+            if exclude_user_id is not None and member.user_id == exclude_user_id:
+                continue
+            publish_event(member.user_id, event_type, data)
+    except Exception as e:
+        logger.error(f"publish_to_conversation failed for conversation {conversation_id}: {e}")
+
+
+# ---------------------------------------------------------------------------
+# Presence helpers
+# ---------------------------------------------------------------------------
+
+def set_user_online(user_id):
+    """Mark user as online with 60s TTL via SETEX."""
+    if not _redis_available or _redis_client is None:
+        return
+    try:
+        _redis_client.setex(f"user:{user_id}:online", 60, "1")
+    except Exception as e:
+        logger.error(f"set_user_online failed for user {user_id}: {e}")
+
+
+def is_user_online(user_id):
+    """Return True if user has an active online key in Redis."""
+    if not _redis_available or _redis_client is None:
+        return False
+    try:
+        return _redis_client.exists(f"user:{user_id}:online") > 0
+    except Exception as e:
+        logger.error(f"is_user_online failed for user {user_id}: {e}")
+        return False
+
+
+def get_user_last_seen(user_id):
+    """Return the stored last_seen ISO timestamp string, or None."""
+    if not _redis_available or _redis_client is None:
+        return None
+    try:
+        return _redis_client.get(f"user:{user_id}:last_seen")
+    except Exception as e:
+        logger.error(f"get_user_last_seen failed for user {user_id}: {e}")
+        return None
+
+
+def update_last_seen(user_id):
+    """Set user:{user_id}:last_seen to current UTC ISO timestamp."""
+    if not _redis_available or _redis_client is None:
+        return
+    try:
+        ts = datetime.now(timezone.utc).isoformat()
+        _redis_client.set(f"user:{user_id}:last_seen", ts)
+    except Exception as e:
+        logger.error(f"update_last_seen failed for user {user_id}: {e}")