feat: merge cartsnitch/api into api/ subdirectory

Consolidate API gateway service into monorepo.
Squashed from https://github.com/cartsnitch/api main (89bacb1).

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

src/cartsnitch_api/config.py

@@ -0,0 +1,51 @@
+import base64
+
+from pydantic import 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"
+    redis_url: str = "redis://localhost:6379/0"
+
+    jwt_secret_key: str = "change-me-in-production"
+    jwt_algorithm: str = "HS256"
+    jwt_access_token_expire_minutes: int = 15
+    jwt_refresh_token_expire_days: int = 7
+
+    service_key: str = "change-me-in-production"
+    # Valid Fernet key for local dev — MUST be overridden in production
+    fernet_key: str = "7reF42nmTwbdN21PBoubGp7h_FU8qSimstmlaMLoRK8="
+
+    cors_origins: list[str] = ["http://localhost:3000", "https://cartsnitch.com"]
+
+    receiptwitness_url: str = "http://receiptwitness:8001"
+    stickershock_url: str = "http://stickershock:8002"
+    clipartist_url: str = "http://clipartist:8003"
+    shrinkray_url: str = "http://shrinkray:8004"
+
+    rate_limit_requests: int = 60
+    rate_limit_window_seconds: int = 60
+    rate_limit_enabled: bool = True
+
+    @model_validator(mode="after")
+    def validate_fernet_key(self):
+        """Validate fernet_key is a valid 32-byte url-safe base64 key at startup."""
+        try:
+            decoded = base64.urlsafe_b64decode(self.fernet_key.encode())
+            if len(decoded) != 32:
+                raise ValueError
+        except Exception:
+            raise ValueError(
+                "CARTSNITCH_FERNET_KEY must be a valid Fernet key "
+                "(32 bytes, url-safe base64 encoded). "
+                "Generate one with: python -c "
+                "'from cryptography.fernet import Fernet; "
+                "print(Fernet.generate_key().decode())'"
+            ) from None
+        return self
+
+
+settings = Settings()