fix(dm): Fix unread markers by deduping retries in all endpoints

Extracted dedup_retry_messages() helper (300s window, was 120s which
was too tight) and applied it in three places:
- GET /api/dm/messages - already had inline dedup, now uses helper
- get_dm_conversations() - fixes last_message_timestamp inflation
- GET /api/dm/updates - was missing dedup entirely, counted retries
  as unread messages (root cause of persistent unread markers)

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

app/meshcore/parser.py

@@ -559,6 +559,26 @@ def read_dm_messages(
     return messages, pubkey_to_name
 
 
+def dedup_retry_messages(messages: List[Dict], window_seconds: int = 300) -> List[Dict]:
+    """Collapse outgoing messages with same text+recipient within a time window.
+
+    Auto-retry sends multiple SENT_MSG entries for the same message.
+    This keeps only the first occurrence and drops duplicates within the window.
+    """
+    deduped = []
+    seen_outgoing = {}  # (recipient, text) -> earliest timestamp
+    for msg in messages:
+        if msg.get('direction') == 'outgoing':
+            key = (msg.get('recipient', ''), msg.get('content', ''))
+            ts = msg.get('timestamp', 0)
+            prev_ts = seen_outgoing.get(key)
+            if prev_ts is not None and abs(ts - prev_ts) <= window_seconds:
+                continue
+            seen_outgoing[key] = ts
+        deduped.append(msg)
+    return deduped
+
+
 def get_dm_conversations(days: Optional[int] = 7) -> List[Dict]:
     """
     Get list of DM conversations with metadata.
@@ -582,19 +602,7 @@ def get_dm_conversations(days: Optional[int] = 7) -> List[Dict]:
     """
     messages, pubkey_to_name = read_dm_messages(days=days)
 
-    # Deduplicate outgoing retry messages: collapse same text+recipient within 120s
-    deduped = []
-    seen_outgoing = {}  # (recipient, text) -> earliest timestamp
-    for msg in messages:
-        if msg.get('direction') == 'outgoing':
-            key = (msg.get('recipient', ''), msg.get('content', ''))
-            ts = msg.get('timestamp', 0)
-            prev_ts = seen_outgoing.get(key)
-            if prev_ts is not None and abs(ts - prev_ts) < 120:
-                continue
-            seen_outgoing[key] = ts
-        deduped.append(msg)
-    messages = deduped
+    messages = dedup_retry_messages(messages)
 
     # Build reverse mapping: name -> pubkey_prefix
     name_to_pubkey = {name: pk for pk, name in pubkey_to_name.items()}