feat(/add): time parsing, link uniqueness, admin-only

- Add time parsing (HH:MM format) after date
  Example: /add Fix bug https://github.com/foo/bar april 15 14:30
- Update check_link_unique to return conflicting bounty ID
- Add_bounty now includes bounty ID in duplicate link error
- cmd_add now catches PermissionError and displays admin-only message
- Update usage text and help message
- Fixes #45

core/services.py

@@ -96,21 +96,24 @@ class BountyService:
 
     def check_link_unique(
         self, room_id: int, link: str | None, exclude_bounty_id: int | None = None
-    ) -> bool:
-        """Check if a link is unique within a room (not used by another bounty)."""
+    ) -> int | None:
+        """Check if a link is unique within a room (not used by another bounty).
+
+        Returns the conflicting bounty ID if found, or None if unique/allowed.
+        """
         if not link:
-            return True
+            return None
 
         room_data = self._storage.load(room_id)
         if room_data is None:
-            return True
+            return None
 
         for bounty in room_data.bounties:
             if bounty.deleted_at is not None:
                 continue
             if bounty.link == link and bounty.id != exclude_bounty_id:
-                return False
-        return True
+                return bounty.id
+        return None
 
     def add_bounty(
         self,
@@ -124,8 +127,11 @@ class BountyService:
         if not self.is_admin(room_id, user_id):
             raise PermissionError("Only admins can add bounties.")
 
-        if not self.check_link_unique(room_id, link):
-            raise ValueError("A bounty with this link already exists in this room.")
+        conflicting_id = self.check_link_unique(room_id, link)
+        if conflicting_id is not None:
+            raise ValueError(
+                f"A bounty with this link already exists: #{conflicting_id}"
+            )
 
         room_data = self._storage.load(room_id)
         if room_data is None:
@@ -178,8 +184,10 @@ class BountyService:
         if not self.is_admin(room_id, user_id):
             raise PermissionError("Only admins can edit bounties.")
 
-        if link and not self.check_link_unique(
-            room_id, link, exclude_bounty_id=bounty_id
+        if (
+            link
+            and self.check_link_unique(room_id, link, exclude_bounty_id=bounty_id)
+            is not None
         ):
             raise ValueError("A bounty with this link already exists in this room.")