Fix immersion tracker SQLite timestamp truncation

- Bind epoch ms values as text to avoid libsql numeric truncation
- Update retention, lifetime, and query tests for string timestamps
- Add backlog ticket for the SQLite timestamp bug

backlog/tasks/task-261 - Fix-immersion-tracker-SQLite-timestamp-truncation.md

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+---
+id: TASK-261
+title: Fix immersion tracker SQLite timestamp truncation
+status: In Progress
+assignee: []
+created_date: '2026-03-31 01:45'
+labels:
+  - immersion-tracker
+  - sqlite
+  - bug
+dependencies: []
+references:
+  - src/core/services/immersion-tracker
+priority: medium
+ordinal: 1200
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Current-epoch millisecond values are being truncated by the libsql driver when bound as numeric parameters, which corrupts session, telemetry, lifetime, and rollup timestamps.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [ ] #1 Current-epoch millisecond timestamps persist correctly in session, telemetry, lifetime, and rollup tables
+- [ ] #2 Startup backfill and destroy/finalize flows keep retained sessions and lifetime summaries consistent
+- [ ] #3 Regression tests cover the destroyed-session, startup backfill, and distinct-day/distinct-video lifetime semantics
+<!-- AC:END -->