fix: remove expand_fields from conference list and add server-side pagination

[Boanerges1996]

Closes #17

Summary

• Conference list was taking 56 seconds due to N+1 queries (expand_fields) and no pagination
• Three changes that work together to fix this:

A) Remove expand_fields from conference list

• List view (GET /conferences?appId=...) no longer inlines participants and issues
• Detail view (GET /conferences/<id>) still returns them
• The dashboard already fetches sessions/issues via separate API calls, so the inlined UUIDs were redundant

B) Server-side pagination (opt-in, backward compatible)

• All list endpoints now accept ?limit= and ?offset= params
• When present: returns { "results": [...], "count": 1234 }
• When absent: returns original { "data": [...] } — no breaking change
• Default limit=50, max=200, invalid values return 400
• Applied to: conferences, sessions, participants, events

C) Fix BaseModel.filter() eager materialization

• Removed .exists() call (result was discarded — wasted query)
• Removed for obj in filtered: obj.prepare() loop that loaded ALL rows into memory
• filter() now returns a lazy QuerySet so LIMIT/OFFSET translates to SQL
• prepare() moved into serialize() loop to preserve App.prepare() behavior

Test results (local, 2079 conferences)

Test	Result
No pagination params → { data: [...] }	✅ Backward compatible
?limit=3&offset=0 → { results, count }	✅ 3 results, count=2079
?limit=3&offset=3 → page 2	✅ Different results
Detail view → has participants/issues	✅
List view → no participants/issues	✅ N+1 eliminated
limit=0 / limit=-1 → clamped to 1	✅
limit=abc → 400 error	✅
limit=500 → clamped to 200	✅
Sessions, participants, events with pagination	✅

Files changed

File	Change
app/models/basemodel.py	Lazy queryset in filter()
app/utils.py	prepare() in serialize loop + paginate_and_serialize() helper
app/views/conference_view.py	Remove expand_fields from list, add pagination
app/views/session_view.py	Add pagination
app/views/participant_view.py	Add pagination
app/views/event_view.py	Pass request to retrieve_events, add pagination
app/views/conference_events_view.py	Pass request to retrieve_events