feat(v2): Import archive .msgs files + DB-based message history

Migration now imports all archive files (oldest first) in addition to the
live .msgs file, with deduplication. Archives endpoint and message history
now query SQLite by date instead of reading .msgs files.

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

app/database.py

@@ -223,6 +223,36 @@ class Database:
             rows = conn.execute(query, params).fetchall()
             return [dict(r) for r in rows]
 
+    def get_message_dates(self) -> List[Dict]:
+        """Get distinct dates that have channel messages, with counts.
+        Returns list of {'date': 'YYYY-MM-DD', 'message_count': N}, newest first."""
+        with self._connect() as conn:
+            rows = conn.execute(
+                """SELECT date(timestamp, 'unixepoch', 'localtime') as date,
+                          COUNT(*) as message_count
+                   FROM channel_messages
+                   WHERE timestamp > 0
+                   GROUP BY date
+                   ORDER BY date DESC"""
+            ).fetchall()
+            return [dict(r) for r in rows]
+
+    def get_channel_messages_by_date(self, date_str: str,
+                                      channel_idx: int = None) -> List[Dict]:
+        """Get channel messages for a specific date (YYYY-MM-DD)."""
+        with self._connect() as conn:
+            conditions = ["date(timestamp, 'unixepoch', 'localtime') = ?"]
+            params: list = [date_str]
+
+            if channel_idx is not None:
+                conditions.append("channel_idx = ?")
+                params.append(channel_idx)
+
+            where = " WHERE " + " AND ".join(conditions)
+            query = f"SELECT * FROM channel_messages{where} ORDER BY timestamp ASC"
+            rows = conn.execute(query, params).fetchall()
+            return [dict(r) for r in rows]
+
     def delete_channel_messages(self, channel_idx: int) -> int:
         with self._connect() as conn:
             cursor = conn.execute(