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>

src/cartsnitch_api/config.py

@@ -1,13 +1,16 @@
 import base64
 
-from pydantic import model_validator
+from pydantic import AliasChoices, Field, model_validator
 from pydantic_settings import BaseSettings
 
 
 class Settings(BaseSettings):
     model_config = {"env_prefix": "CARTSNITCH_"}
 
-    database_url: str = "postgresql+asyncpg://cartsnitch:cartsnitch@localhost:5432/cartsnitch"
+    database_url: str = Field(
+        default="postgresql+asyncpg://cartsnitch:cartsnitch@localhost:5432/cartsnitch",
+        validation_alias=AliasChoices("CARTSNITCH_DATABASE_URL", "DATABASE_URL"),
+    )
     redis_url: str = "redis://localhost:6379/0"
 
     jwt_secret_key: str = "change-me-in-production"
@@ -49,5 +52,12 @@ class Settings(BaseSettings):
             ) from None
         return self
 
+    @model_validator(mode="after")
+    def normalize_database_url(self):
+        """Normalize postgresql:// → postgresql+asyncpg:// for the asyncpg driver."""
+        if self.database_url.startswith("postgresql://"):
+            self.database_url = self.database_url.replace("postgresql://", "postgresql+asyncpg://", 1)
+        return self
+
 
 settings = Settings()