Migrate Postgres timestamps to `TIMESTAMP WITH TIME ZONE`

[peteski22]

Part of #257 — Phase 3 step (2).
RFC: #275.

Blocked by the datetime-at-store-boundary Phase 3 child.

Scope

Migrate Postgres timestamp columns from TEXT to
TIMESTAMP WITH TIME ZONE. Includes Alembic migration with data
conversion (parsing the existing ISO strings). SQLite continues to
store ISO strings for now — no SQLite migration in this child.

Definition of Done

• Alembic migration runs on a populated testcontainers Postgres in
  CI and converts data without loss.
• Round-trip tests confirm datetime values come back identically
  on both backends.
• Full test suite green.
• Lint clean.
• Operator-facing note in deployment docs: this migration rewrites
  data in knowledge_units (timestamps); plan a maintenance window
  for large tables.