fix(config): rifiuta i segreti di default fuori da dev/test (#33, parte A2)

Nuova impostazione app_env (env APP_ENV, default "dev"). All'istanza di
Settings, se app_env non è "dev"/"test" e SERVER_SECRET_KEY o
DB_PASSWORD sono ancora il valore committato, RuntimeError che nomina
la variabile. Dev e test continuano a partire senza .env: il default
"dev" non fa mai scattare la guardia.

Le parti frontend (flask_app/config.py) e deploy (docker-compose.yml)
di questa issue sono degli altri lotti (B2, C2).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/config.py

@@ -1,22 +1,37 @@
 """TieMeasureFlow Server Configuration."""
 from pathlib import Path
+
+from pydantic import model_validator
 from pydantic_settings import BaseSettings
 
+# Committed defaults. Fine for dev/test (SQLite in-memory, nothing exposed); a
+# real deployment that still has these is running on secrets anyone can read
+# in this file's git history.
+_DEFAULT_SECRET_KEY = "change-this-to-a-random-secret-key"
+_DEFAULT_DB_PASSWORD = "change_me_in_production"
+
 
 class Settings(BaseSettings):
     """Application settings loaded from environment variables."""
 
+    # Deployment environment. "dev" and "test" skip the production-only guard
+    # below; anything else (e.g. "production") is treated as a real
+    # deployment. Local dev and the test suite must keep starting with no
+    # .env at all, which is why this defaults to "dev" rather than requiring
+    # the variable.
+    app_env: str = "dev"
+
     # Database
     db_host: str = "localhost"
     db_port: int = 3306
     db_name: str = "tiemeasureflow"
     db_user: str = "tmflow"
-    db_password: str = "change_me_in_production"
+    db_password: str = _DEFAULT_DB_PASSWORD
 
     # Server
     server_host: str = "0.0.0.0"
     server_port: int = 8000
-    server_secret_key: str = "change-this-to-a-random-secret-key"
+    server_secret_key: str = _DEFAULT_SECRET_KEY
     server_cors_origins: str = "http://localhost:5000"
     # Worker processes (e.g. `uvicorn --workers N`). Each rate-limit bucket lives
     # in process memory, so this is how RateLimitMiddleware divides the
@@ -86,5 +101,27 @@ class Settings(BaseSettings):
         "extra": "ignore",
     }
 
+    @model_validator(mode="after")
+    def _refuse_default_secrets_outside_dev(self) -> "Settings":
+        """Refuse to boot with a committed default secret in a real deployment.
+
+        Only fires when APP_ENV names an actual deployment (not "dev"/"test"),
+        so local development and the test suite keep starting with no .env at
+        all.
+        """
+        if self.app_env in ("dev", "test"):
+            return self
+        if self.server_secret_key == _DEFAULT_SECRET_KEY:
+            raise RuntimeError(
+                "SERVER_SECRET_KEY is still the committed default value; "
+                "set it before running with APP_ENV=" + self.app_env
+            )
+        if self.db_password == _DEFAULT_DB_PASSWORD:
+            raise RuntimeError(
+                "DB_PASSWORD is still the committed default value; "
+                "set it before running with APP_ENV=" + self.app_env
+            )
+        return self
+
 
 settings = Settings()