feat: add /trip slash commands + add trips & itinerary system

New /trips router with full CRUD, attendee management, and nested
events. Events validate date is within parent trip range and inherit
trip location when not explicitly set. Leaving a trip cascades
removal from all its events.

New TripCommands cog with /trip create, list, view, delete, join,
leave and /trip event add, remove, join, leave. Event autocomplete
is scoped to the selected trip. Enforces must-be-on-trip rule for
event joins with a clear error message.

drb-c2-core/app/models.py

@@ -134,3 +134,29 @@ class AlertEvent(BaseModel):
     transcript_snippet: Optional[str] = None
     triggered_at: Optional[datetime] = None
     acknowledged: bool = False
+
+
+# ---------------------------------------------------------------------------
+# Trips
+# ---------------------------------------------------------------------------
+
+class TripCreate(BaseModel):
+    name: str
+    location: str
+    maps_link: Optional[str] = None
+    start_date: str  # YYYY-MM-DD
+    end_date: str    # YYYY-MM-DD
+
+
+class TripEventCreate(BaseModel):
+    title: str
+    date: str               # YYYY-MM-DD, must fall within parent trip range
+    time: Optional[str] = None          # HH:MM (24h)
+    location: Optional[str] = None      # inherits trip location if None
+    maps_link: Optional[str] = None
+    notes: Optional[str] = None
+
+
+class AttendeeAction(BaseModel):
+    discord_user_id: str
+    discord_username: Optional[str] = None