fix(store): add missing "lt" operator to PG filter parser

[gustavosbarreto]

What

Added the missing "lt" (less than) operator to the PostgreSQL
filter property parser, which was silently discarding the filter
condition used by the device cleanup cron job.

Why

The device cleanup cron (runs daily at 2 AM UTC) filters devices
with removed_at < 30 days ago. Since the PG filter parser had
no handler for the "lt" operator, it fell through to the default
case returning an empty condition without error. The cleanup then
ran unfiltered, deleting all devices. Agents reconnecting
afterward created new pending devices, causing every accepted
device to revert to pending status every morning.

Fixes #6134

Changes

• store/pg/internal/filters.go: added fromLt() with support
  for numeric types, time.Time, and string values
• store/pg/internal/filters.go: added time.Time case to the
  existing fromGt() for consistency

[github-actions]

Claude finished @gustavosbarreto's task in 1m 46s —— View job

---

Code Review Complete

Reviewed 1 file across code quality, security, testing, Go patterns, and architecture — no issues found. The code looks good as-is.

The new fromLt() correctly mirrors the existing fromGt() with < instead of >, handles the same type set (numeric, time.Time, string-to-number), uses parameterized queries via bun's ? placeholders, and has no cross-repo impact (cloud's MongoDB filter parser is independent and already supports "lt").

---

To request another review round, comment /review.