feat(meetings): add summary + summary_state fields to the meeting model

[Rahulkaushik01]

What & why

Adds storage for an AI-generated meeting title + summary on the shared Bot meeting
record, and exposes it in the member-facing MeetingSerializer. This is the schema slice
of the summaries feature — storage + read exposure only; generation lands in a later
phase. One summary per meeting (shared by all viewers), covering bot meetings and local
recordings alike (same model, split by session_type).

No behaviour change: every meeting starts summary=null, summary_state=pending, and
nothing generates or displays it yet.

Stacked on # (Azure OpenAI credential). Review/merge after it, or retarget to dev.

Changes (per file)

• bots/models.py — SummaryStates TextChoices (pending/generating/done/failed/skipped);
  summary (TextField, null), summary_state (CharField, choices, default pending),
  summary_generated_at (DateTimeField, null) on Bot.
• bots/meetings_serializers.py — expose summary + summary_state as plain fields. They
  are direct columns on the already-fetched row, so the serializer's no-N+1 property is
  preserved (still guarded by TestMeetingListQueryCount).
• bots/migrations/0096_bot_summary_bot_summary_generated_at_and_more.py — additive,
  nullable/defaulted, depends on 0095; no backfill.
• bots/tests/test_meeting_summary_fields.py — 4 tests.

Behaviour / lifecycle

• Fields are read-only over the API (the serializer is output-only).
• Deletion: a full meeting delete removes the row (and its summary). Clearing the summary on
  a transcript-only wipe (delete_data) is intentionally deferred to the generation phase,
  where it ships with the code that writes summaries — harmless until then (always null).

Testing

• makemigrations --check → No changes detected; migration applies + reverses cleanly.
• python manage.py test bots.tests.test_meeting_summary_fields → 4 passed; meetings-list +
  journey suites → pass (no regression; query-count guard still green).
• ruff check / ruff format --check → clean.

Notes

• No new dependencies. No write path opened. summary_state values are named choices (no
  magic strings).