perf(conversion): source start/end events from profile_event_summary_mv when eligible

[ayushjhanwar-png]

Summary

Conversion charts on plain event names (no per-event filters, no breakdown / hold properties) now source the start_events_raw and end_events_raw CTEs from the existing `profile_event_summary_mv` instead of scanning the raw `events` table.

The MV is already pre-aggregated by `(project_id, profile_id, name, event_date)` with `first_event_time` stored as an AggregateFunction — exactly the shape downstream wants (`min(created_at)` per profile per day). For high-traffic projects on wide windows, pulling from the MV skips hundreds of millions of rows.

Eligibility

A single event's CTE uses the MV path only when:

• No per-event filters (the MV doesn't store property values).
• No extra columns (breakdown / hold properties only live on `events`).
• `groupCol === 'profile_id'` (the MV is profile-keyed; session-conversions stay on the events table).

Otherwise the existing `events`-table path is used unchanged. `session_id` is emitted as `''` from the MV branch — downstream `any(session_id)` accepts it and current callers don't read it when grouping by `profile_id`.

Measured impact

1-month shortreels `appOpen` conversion:

Path	Read	Duration
events table	~28 GiB	10-40s (frequent 40s timeouts)
MV	~6 GiB	~600 ms

Numbers match the `events` table exactly. Verified side-by-side via `uniq(profile_id)` and `countMerge(event_count)`:

```
events: 571410 profiles, 2244741 events
MV: 571410 profiles, 2244741 events
```

Test plan

• Plain conversion (e.g. `appOpen → onboarding_complete`) on a 7d / 30d window: returns same daily counts as before, finishes in <2s.
• Conversion with a per-event filter (e.g. `appOpen [country = US]`): still uses the events-table path, behaviour unchanged.
• Conversion with a breakdown (e.g. by `os`): still uses the events-table path, behaviour unchanged.
• Conversion with a hold property (e.g. `hold by showId`): still uses the events-table path, behaviour unchanged.
• Conversion with `funnelGroup: session_id`: still uses the events-table path, behaviour unchanged.
• Conversion where start / end is a custom event: the `if (customEvent)` branch above is taken — MV path is bypassed.

Notes

• Doesn't materially help correctness, only perf — but conversions on plain event names are the common case (the slow shortreels appOpen → ... example that triggered this PR is exactly that).
• The MV used (`profile_event_summary_mv`) is the existing one fed by the standard insertion path; no new MV to maintain.