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/routers/trips.py

@@ -0,0 +1,150 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional
+from fastapi import APIRouter, HTTPException
+from app.models import TripCreate, TripEventCreate, AttendeeAction
+from app.internal import firestore as fstore
+
+router = APIRouter(prefix="/trips", tags=["trips"])
+
+
+@router.get("")
+async def list_trips():
+    return await fstore.collection_list("trips")
+
+
+@router.post("")
+async def create_trip(body: TripCreate):
+    if body.end_date < body.start_date:
+        raise HTTPException(400, "end_date must be on or after start_date.")
+    trip_id = str(uuid.uuid4())
+    now = datetime.now(timezone.utc).isoformat()
+    doc = {
+        "trip_id": trip_id,
+        "name": body.name,
+        "location": body.location,
+        "maps_link": body.maps_link,
+        "start_date": body.start_date,
+        "end_date": body.end_date,
+        "attendees": {},  # {discord_user_id: discord_username}
+        "created_at": now,
+    }
+    await fstore.doc_set("trips", trip_id, doc, merge=False)
+    return doc
+
+
+@router.get("/{trip_id}")
+async def get_trip(trip_id: str):
+    trip = await fstore.doc_get("trips", trip_id)
+    if not trip:
+        raise HTTPException(404, f"Trip '{trip_id}' not found.")
+    events = await fstore.collection_list("trip_events", trip_id=trip_id)
+    events.sort(key=lambda e: (e["date"], e.get("time") or ""))
+    return {**trip, "events": events}
+
+
+@router.delete("/{trip_id}")
+async def delete_trip(trip_id: str):
+    trip = await fstore.doc_get("trips", trip_id)
+    if not trip:
+        raise HTTPException(404, f"Trip '{trip_id}' not found.")
+    events = await fstore.collection_list("trip_events", trip_id=trip_id)
+    for e in events:
+        await fstore.doc_delete("trip_events", e["event_id"])
+    await fstore.doc_delete("trips", trip_id)
+    return {"ok": True}
+
+
+@router.post("/{trip_id}/join")
+async def join_trip(trip_id: str, body: AttendeeAction):
+    trip = await fstore.doc_get("trips", trip_id)
+    if not trip:
+        raise HTTPException(404, f"Trip '{trip_id}' not found.")
+    attendees = trip.get("attendees", {})
+    attendees[body.discord_user_id] = body.discord_username or body.discord_user_id
+    await fstore.doc_update("trips", trip_id, {"attendees": attendees})
+    return {"ok": True, "attendees": attendees}
+
+
+@router.post("/{trip_id}/leave")
+async def leave_trip(trip_id: str, body: AttendeeAction):
+    trip = await fstore.doc_get("trips", trip_id)
+    if not trip:
+        raise HTTPException(404, f"Trip '{trip_id}' not found.")
+    attendees = trip.get("attendees", {})
+    attendees.pop(body.discord_user_id, None)
+    await fstore.doc_update("trips", trip_id, {"attendees": attendees})
+    # cascade: remove from all events in this trip
+    events = await fstore.collection_list("trip_events", trip_id=trip_id)
+    for e in events:
+        event_attendees = e.get("attendees", {})
+        if body.discord_user_id in event_attendees:
+            event_attendees.pop(body.discord_user_id)
+            await fstore.doc_update("trip_events", e["event_id"], {"attendees": event_attendees})
+    return {"ok": True}
+
+
+@router.post("/{trip_id}/events")
+async def create_event(trip_id: str, body: TripEventCreate):
+    trip = await fstore.doc_get("trips", trip_id)
+    if not trip:
+        raise HTTPException(404, f"Trip '{trip_id}' not found.")
+    if not (trip["start_date"] <= body.date <= trip["end_date"]):
+        raise HTTPException(
+            400,
+            f"Event date {body.date} is outside the trip range "
+            f"{trip['start_date']} – {trip['end_date']}.",
+        )
+    event_id = str(uuid.uuid4())
+    now = datetime.now(timezone.utc).isoformat()
+    doc = {
+        "event_id": event_id,
+        "trip_id": trip_id,
+        "title": body.title,
+        "date": body.date,
+        "time": body.time,
+        "location": body.location if body.location is not None else trip["location"],
+        "location_inherited": body.location is None,
+        "maps_link": body.maps_link,
+        "notes": body.notes,
+        "attendees": {},
+        "created_at": now,
+    }
+    await fstore.doc_set("trip_events", event_id, doc, merge=False)
+    return doc
+
+
+@router.delete("/{trip_id}/events/{event_id}")
+async def delete_event(trip_id: str, event_id: str):
+    event = await fstore.doc_get("trip_events", event_id)
+    if not event or event.get("trip_id") != trip_id:
+        raise HTTPException(404, f"Event '{event_id}' not found in trip '{trip_id}'.")
+    await fstore.doc_delete("trip_events", event_id)
+    return {"ok": True}
+
+
+@router.post("/{trip_id}/events/{event_id}/join")
+async def join_event(trip_id: str, event_id: str, body: AttendeeAction):
+    trip = await fstore.doc_get("trips", trip_id)
+    if not trip:
+        raise HTTPException(404, f"Trip '{trip_id}' not found.")
+    if body.discord_user_id not in trip.get("attendees", {}):
+        raise HTTPException(403, "You must join the trip before joining an event.")
+    event = await fstore.doc_get("trip_events", event_id)
+    if not event or event.get("trip_id") != trip_id:
+        raise HTTPException(404, f"Event '{event_id}' not found in trip '{trip_id}'.")
+    attendees = event.get("attendees", {})
+    attendees[body.discord_user_id] = body.discord_username or body.discord_user_id
+    await fstore.doc_update("trip_events", event_id, {"attendees": attendees})
+    return {"ok": True, "attendees": attendees}
+
+
+@router.post("/{trip_id}/events/{event_id}/leave")
+async def leave_event(trip_id: str, event_id: str, body: AttendeeAction):
+    event = await fstore.doc_get("trip_events", event_id)
+    if not event or event.get("trip_id") != trip_id:
+        raise HTTPException(404, f"Event '{event_id}' not found in trip '{trip_id}'.")
+    attendees = event.get("attendees", {})
+    attendees.pop(body.discord_user_id, None)
+    await fstore.doc_update("trip_events", event_id, {"attendees": attendees})
+    return {"ok": True}