Add edge-case tests for failures, timeouts, and concurrent access

[vladiant]

Summary

Extends test coverage with 67 new edge-case tests targeting failure paths, timeout behavior, and concurrent access patterns across all architecture layers.

Changes

File	Tests	Focus
test_edge_cases.py	22	Use case failure/timeout/concurrency
test_edge_cases.py	27	Storage, processor, cache thread-safety
test_api_edge_cases.py	18	API error responses, validation boundaries

What's tested

Failure paths

• Cascading failures in ProcessImageUseCase (thumbnail cleanup OSError, processor ValueError, save failures at each stage)
• Partial storage cleanup failures in retention sweeps (error counting)
• Storage/repository errors propagating correctly in upload and get flows
• Cache behavior when inner repository raises (stale entries, invalidation ordering)

Timeouts

• asyncio.timeout wrapping slow processing, storage, and retention operations

Concurrent access

• Pipeline with mixed success/failure/not-found results, all-failure batches, empty batches, serialized execution
• Thread-safe cache operations under contention (set/get, set/invalidate, eviction pressure, concurrent clear)
• Concurrent file storage with same filename producing unique paths
• Concurrent deletes of the same file (exactly-once semantics)
• Concurrent get_by_id cache misses (thundering herd)

API validation boundaries

• Empty/corrupt/non-image uploads, BMP content type rejection
• Exactly-at-limit tag count (20), invalid UUID format
• Negative offset, zero/over-max limit, empty list results
• Internal 500 errors from use case exceptions
• Retention sweep error reporting and DB deadlock propagation

Version bump

PATCH: 2.0.0 → 2.0.1 (tests only, no API or behavior changes)

Checklist

• ruff check — all clean
• ruff format — all formatted
• mypy — success, no issues
• pytest — 168/168 passing (67 new + 101 existing)
• CHANGELOG.md updated
• Version bumped in pyproject.toml