fix: fail closed on ambiguous delivery reports

[Justinabox]

Summary

Closes #294.

• Correlate a delivery report only when its (reference, recipient) identifies exactly one outbound SMS.
• Persist ambiguous reports with message_id = None without changing either outbound SMS status.
• Ignore matching inbound SMS records during outbound delivery-report correlation.
• Cover in-memory ambiguity, SQLite close/reopen persistence, unique matching, and inbound/outbound boundary behavior.

Verification

• git diff --check
• PYTHONPATH=. uv run --no-project --with pytest --with pytest-asyncio --with pytest-aiohttp --with pyserial-asyncio --with aiosqlite pytest tests/test_sms_store.py -q → 59 passed
• PYTHONPATH=. uv run --no-project --with pytest --with pytest-asyncio --with pytest-aiohttp --with pyserial-asyncio --with aiosqlite pytest tests/ -q → 1102 passed

Risk

Intentional fail-closed behavior leaves repeated-reference reports unlinked for operator inspection rather than guessing which outbound message to update. No timestamp heuristics or modem/hardware behavior changed.