test: add characterization tests for core modules

33 tests covering env/io helpers, config parsers, pipeline meta I/O,
worker inbox logic, and FastAPI smoke routes. No gmic/rembg subprocesses.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/conftest.py

@@ -0,0 +1,26 @@
+"""Shared test fixtures."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture
+def data_dir(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
+    """Point config path constants at a temporary data directory."""
+    variants = tmp_path / "variants"
+    intermediates = tmp_path / "intermediates"
+    meta = tmp_path / "meta"
+    inbox = tmp_path / "incoming"
+    for directory in (variants, intermediates, meta, inbox):
+        directory.mkdir()
+
+    monkeypatch.setattr("app.config.DATA_DIR", tmp_path)
+    monkeypatch.setattr("app.config.VARIANTS_DIR", variants)
+    monkeypatch.setattr("app.config.INTERMEDIATES_DIR", intermediates)
+    monkeypatch.setattr("app.config.META_DIR", meta)
+    monkeypatch.setattr("app.config.INBOX_DIR", inbox)
+    monkeypatch.setattr("app.config.PROCESSED_FILE", tmp_path / "processed.json")
+    return tmp_path