Convert timestamps to `datetime` at the store boundary

[peteski22]

Part of #257 — Phase 3 step (1).
RFC: #275.

Blocked by the PostgresStore implementation Phase 2 child.

Scope

Convert timestamp columns to datetime objects at the store boundary
on both backends. Single conversion point; callers stop seeing
strings. Update callers that relied on string representation.

The motivation (per RFC #275): the current TEXT-typed timestamps
prevent Postgres from indexing date ranges or doing interval
arithmetic. The "parity" between backends only existed because the
existing code was stringly typed.

Definition of Done

• TDD: failing tests for the new typed boundary on each affected
  method, then the conversion code.
• Caller updates in the same PR — no half-typed surface.
• Full test suite green on both backends.
• Lint clean.