fix(db): guard string_to_datetime against non-string input

[clewdotcc]

Problem

pyrekordbox.masterdb.models.string_to_datetime (also exposed as pyrekordbox.db6.tables.string_to_datetime via the back-compat re-export) calls value.endswith("Z") without type-guarding value. Rekordbox 6 has been observed in the wild to store non-string values (specifically integer 0) in DateTime columns — for example DjmdAlbum.created_at / updated_at.

When this happens, the function raises AttributeError: 'int' object has no attribute 'endswith' at SQLAlchemy result-set materialization time. This is not a recoverable per-row error — a single corrupt row crashes the entire query: session.query(DjmdAlbum).all(), .first(), etc. all fail.

Reproduction

>>> from pyrekordbox.masterdb.models import string_to_datetime
>>> string_to_datetime(0)
Traceback (most recent call last):
  ...
AttributeError: 'int' object has no attribute 'endswith'

Fix

Type-guard the function: return None for non-string input. This treats malformed DateTime columns as "no date" rather than crashing the consuming query.

This is consistent with the existing DateTime.process_result_value behavior, which already declares -> Optional[datetime] and returns None when the raw column value is falsy. The return-type annotation of string_to_datetime is widened from datetime to Optional[datetime] to honestly describe the new None return path; the only in-repo caller (DateTime.process_result_value) already handles Optional[datetime].

Before / After

# Before
>>> string_to_datetime(0)
AttributeError: 'int' object has no attribute 'endswith'

# After
>>> string_to_datetime(0)  # returns None — treated as missing value

Behavior on valid string input is unchanged. Behavior on empty string "" is also unchanged (still raises ValueError from datetime.fromisoformat(""), same as before) — in practice the DateTime TypeDecorator's truthy guard at process_result_value already short-circuits this path, so the empty-string semantic is irrelevant in production but is preserved here for strict backward-compat.

Tests

Added test_string_to_datetime_returns_none_for_non_string_input in tests/test_masterdb.py, parametrized over 0, None, and a larger integer. The test fails on master (raises AttributeError) and passes with this patch.

Scope

Limited to pyrekordbox/masterdb/models.py. Side-find: pyrekordbox/devicelib_plus/models.py has a byte-identical string_to_datetime with the same vulnerability. I kept it out of this PR to keep the review focused, but happy to extend the PR or open a parallel one — whichever you prefer.

Backward compatibility

• Valid ISO string inputs: behavior unchanged.
• Empty string: behavior unchanged (still raises ValueError).
• Non-string inputs (int, None, ...): previously crashed, now return None.
• Return type widens from datetime to Optional[datetime]. The only in-repo caller already handles Optional[datetime]; external callers that rely on a non-None return for non-string inputs were already encountering AttributeError, so this is a strict improvement.