fix(service): move StartLimit* keys to [Unit] section

Modern systemd (>= 230) requires StartLimitIntervalSec= and
StartLimitBurst= in [Unit]. In [Service] they are silently ignored,
so the observer's restart rate-limit never took effect and an
observer crash loop was not throttled. Reference: systemd.unit(5).

This eliminates the "Unknown key 'StartLimitIntervalSec'" and
"Unknown key 'StartLimitBurst'" warnings emitted by systemd when
loading the unit.

Existing installs pick up the fix on their next `make install-service`
run — no migration logic required.

Out of scope / follow-ups:
- solstone-tmux's unit template has the identical bug (separate lode).
- tests/test_sync.py::TestQuarantineZeroByte::test_zero_byte_segment_quarantined
fails on base as well; unrelated pre-existing mismatch between sync
quarantine cleanup and its test expectations.

Co-Authored-By: OpenAI Codex <codex@openai.com>