feat(rollup): add models and write path [1/3] (#13796)

## PR#1: Reporting events rollup — model and write path

Reporting queries currently hit the `reporting_events` table directly.
This works, but the table grows linearly with event volume, and
aggregation queries (counts, averages over date ranges) get
progressively slower as accounts age.

This PR introduces a pre-aggregated `reporting_events_rollups` table
that stores daily per-metric, per-dimension (account/agent/inbox)
totals. The write path is intentionally decoupled from the read path —
rollup rows are written inline from the event listener via upsert, and a
backfill service exists to rebuild historical data from raw events.
Nothing reads from this table yet.

The write path activates when an account has a `reporting_timezone` set
(new account setting). The `reporting_events_rollup` feature flag
controls only the future read path, not writes — so rollup data
accumulates silently once timezone is configured. A `MetricRegistry`
maps raw event names to rollup column semantics in one place, keeping
the write and (future) read paths aligned.

### What changed

- Migration for `reporting_events_rollups` with a unique composite index
for upsert
- `ReportingEventsRollup` model
- `reporting_timezone` account setting with IANA timezone validation
- `MetricRegistry` — single source of truth for event-to-metric mappings
- `RollupService` — real-time upsert from event listener
- `BackfillService` — rebuilds rollups for a given account + date from
raw events
- Rake tasks for interactive backfill and timezone setup
- `reporting_events_rollup` feature flag (disabled by default)

### How to test

1. Set a `reporting_timezone` on an account
(`Account.first.update!(reporting_timezone: 'Asia/Kolkata')`)
2. Resolve a conversation or trigger a first response
3. Check `ReportingEventsRollup.where(account_id: ...)` — rows should
appear
4. Run backfill: `bundle exec rake reporting_events_rollup:backfill` and
verify historical data populates

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/helpers/timezone_helper.rb

@@ -1,4 +1,10 @@
 module TimezoneHelper
+  def timezone_name_from_params(timezone, offset)
+    return timezone if timezone.present? && ActiveSupport::TimeZone[timezone].present?
+
+    timezone_name_from_offset(offset)
+  end
+
   # ActiveSupport TimeZone is not aware of the current time, so ActiveSupport::Timezone[offset]
   # would return the timezone without considering day light savings. To get the correct timezone,
   # this method uses zone.now.utc_offset for comparison as referenced in the issues below