fix(dbreads): use timezone-aware now() for stats windows

[Harsh63870]

Summary

Fixes #1541.

Several reporting and rollout-policy queries compared timestamptz columns (last_check_for_updates, last_update_granted_ts) to now() at time zone 'utc'. That expression returns a timestamp without time zone. When Postgres compares it back to timestamptz, it converts using the session timezone, so the window shifts by the UTC offset whenever the DB session is not UTC.

That skews:

• Dashboard counts (GetGroupInstancesStats, GetGroupVersionBreakdown, app instance counts, instance list filters)
• Rollout enforcement (GetGroupUpdatesStats → updates_in_progress / updates_timed_out / period rate limits), which can pause updates early (safe mode) or hold instances too long

Timeline queries in the same package already used plain now(), so two reporting paths reading the same columns disagreed.

Changes

1. Query fix: replace now() at time zone 'utc' with timezone-aware now() in:
   • backend/pkg/api/internal/dbreads/applications.go
   • backend/pkg/api/internal/dbreads/instances.go
   • backend/pkg/api/internal/dbreads/groups.go
2. Defense in depth: append TimeZone=UTC to the DB URL on connect when the operator has not already set a timezone (withUTCSessionTimezone in api.go). Explicit TimeZone / timezone / options=-c TimeZone=... are left alone.
3. Tests:
   • TestGroupInstanceStatsStableAcrossSessionTimezones — same 1-day window returns 3 under UTC, America/New_York, and Asia/Kolkata
   • TestWithUTCSessionTimezone — DSN helper behavior
4. CHANGELOG Unreleased bugfix entry

Test plan

• cd backend && go test ./pkg/api/ -run 'TestWithUTCSessionTimezone|TestGroupInstanceStatsStableAcrossSessionTimezones'
• cd backend && go test ./pkg/api/ -run 'TestGetGroup|TestRegisterInstance|TestGetInstances'
• Optional: on a Postgres with SET TIME ZONE 'Asia/Kolkata', confirm group instance totals match UTC before/after

Notes

Sample fixture SQL (sample_data.sql) still uses now() at time zone 'utc' for inserts only; those are not the comparison bug described in #1541 and were left unchanged to keep this PR focused.

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[Harsh63870]

@ervcz , resolved the merge conflict in backend/pkg/api/api.go against upstream/main, upstream had relocated nowUTC to pkg/api/runtime, so this branch now keeps only withUTCSessionTimezone and updates the timezone regression test to use runtimeSvc / runtime.NewInstanceApplication, no changes to the original fix logic, just adapted to the new location...