fix(api): accept DATABASE_URL as fallback for shared DB with auth service

API config.py now reads CARTSNITCH_DATABASE_URL first, falls back to
DATABASE_URL (which the infra K8s overlay sets for all pods), and finally
falls back to the hardcoded default. Also normalizes plain postgresql://
to postgresql+asyncpg:// for the asyncpg driver.

Fixes CAR-510.
Co-Authored-By: Paperclip <noreply@paperclip.ing>

tests/test_config.py

@@ -0,0 +1,48 @@
+"""Tests for Settings config, specifically the database_url env var fallback."""
+
+import os
+
+from cartsnitch_api.config import Settings
+
+
+def test_database_url_prefers_cartsnitch_prefix():
+    """CARTSNITCH_DATABASE_URL takes precedence over DATABASE_URL."""
+    env = {
+        "CARTSNITCH_DATABASE_URL": "postgresql+asyncpg://user1:pass1@host1:5432/db1",
+        "DATABASE_URL": "postgresql://user2:pass2@host2:5432/db2",
+    }
+    settings = Settings(**env)
+    assert settings.database_url == "postgresql+asyncpg://user1:pass1@host1:5432/db1"
+
+
+def test_database_url_falls_back_to_database_url():
+    """When CARTSNITCH_DATABASE_URL is absent, DATABASE_URL is accepted."""
+    env = {
+        "DATABASE_URL": "postgresql://user:pass@dbhost:5432/mydb",
+    }
+    settings = Settings(**env)
+    assert settings.database_url == "postgresql+asyncpg://user:pass@dbhost:5432/mydb"
+
+
+def test_database_url_normalizes_plain_postgresql_prefix():
+    """DATABASE_URL with plain postgresql:// is normalized to postgresql+asyncpg://."""
+    env = {
+        "DATABASE_URL": "postgresql://cartsnitch:cartsnitch@localhost:5432/cartsnitch",
+    }
+    settings = Settings(**env)
+    assert settings.database_url == "postgresql+asyncpg://cartsnitch:cartsnitch@localhost:5432/cartsnitch"
+
+
+def test_database_url_preserves_asyncpg_prefix():
+    """CARTSNITCH_DATABASE_URL with postgresql+asyncpg:// is left unchanged."""
+    env = {
+        "CARTSNITCH_DATABASE_URL": "postgresql+asyncpg://cartsnitch:cartsnitch@localhost:5432/cartsnitch",
+    }
+    settings = Settings(**env)
+    assert settings.database_url == "postgresql+asyncpg://cartsnitch:cartsnitch@localhost:5432/cartsnitch"
+
+
+def test_database_url_default():
+    """When neither env var is set, the hardcoded default is used."""
+    settings = Settings()
+    assert settings.database_url == "postgresql+asyncpg://cartsnitch:cartsnitch@localhost:5432/cartsnitch"