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/test_config.py

@@ -0,0 +1,30 @@
+"""Tests for config helpers."""
+
+from __future__ import annotations
+
+from app import config
+from app.config import _float_env, _int_env
+
+
+def test_int_env_valid(monkeypatch) -> None:
+    monkeypatch.setenv("TEST_INT", "42")
+    assert _int_env("TEST_INT", 0) == 42
+
+
+def test_int_env_invalid_falls_back(monkeypatch) -> None:
+    monkeypatch.setenv("TEST_INT", "nope")
+    assert _int_env("TEST_INT", 7) == 7
+
+
+def test_float_env_valid(monkeypatch) -> None:
+    monkeypatch.setenv("TEST_FLOAT", "2.5")
+    assert _float_env("TEST_FLOAT", 0.0) == 2.5
+
+
+def test_supported_extensions() -> None:
+    assert ".jpg" in config.SUPPORTED_EXTENSIONS
+    assert ".jpeg" in config.SUPPORTED_EXTENSIONS
+
+
+def test_opacity_choices_percentages() -> None:
+    assert all(choice.endswith("%") for choice in config.OPACITY_CHOICES)