feat: enhance error handling and configuration validation

- Added a global exception handler to log unhandled exceptions and return a generic 500 JSON response without exposing details to the client.
- Updated the configuration to validate the `DATABASE_URL` format, ensuring it starts with `sqlite://` or `postgresql://`, and log warnings for invalid formats.
- Introduced safe parsing for numeric environment variables (`HTTP_PORT`, `INIT_DATA_MAX_AGE_SECONDS`) with defaults on invalid values, including logging warnings for out-of-range values.
- Enhanced the duty schedule parser to enforce limits on the number of schedule rows and the length of full names and duty strings, raising appropriate errors when exceeded.
- Updated internationalization messages to include generic error responses for import failures and parsing issues, improving user experience.
- Added unit tests to verify the new error handling and configuration validation behaviors.

duty_teller/api/app.py

@@ -8,7 +8,7 @@ import duty_teller.config as config
 
 from fastapi import Depends, FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import Response
+from fastapi.responses import JSONResponse, Response
 from fastapi.staticfiles import StaticFiles
 from sqlalchemy.orm import Session
 
@@ -42,6 +42,16 @@ def _is_valid_calendar_token(token: str) -> bool:
 app = FastAPI(title="Duty Teller API")
 
 
+@app.exception_handler(Exception)
+def global_exception_handler(request: Request, exc: Exception) -> JSONResponse:
+    """Log unhandled exceptions and return 500 without exposing details to the client."""
+    log.exception("Unhandled exception: %s", exc)
+    return JSONResponse(
+        status_code=500,
+        content={"detail": "Internal server error"},
+    )
+
+
 @app.get("/health", summary="Health check")
 def health() -> dict:
     """Return 200 when the app is up. Used by Docker HEALTHCHECK."""
@@ -106,6 +116,18 @@ class NoCacheStaticMiddleware:
 app.add_middleware(NoCacheStaticMiddleware)
 
 
+# Allowed values for config.js to prevent script injection.
+_VALID_LANGS = frozenset({"en", "ru"})
+_VALID_LOG_LEVELS = frozenset({"debug", "info", "warning", "error"})
+
+
+def _safe_js_string(value: str, allowed: frozenset[str], default: str) -> str:
+    """Return value if it is in allowed set, else default. Prevents injection in config.js."""
+    if value in allowed:
+        return value
+    return default
+
+
 @app.get(
     "/app/config.js",
     summary="Mini App config (language, log level)",
@@ -115,8 +137,8 @@ app.add_middleware(NoCacheStaticMiddleware)
 )
 def app_config_js() -> Response:
     """Return JS assigning window.__DT_LANG and window.__DT_LOG_LEVEL for the webapp. No caching."""
-    lang = config.DEFAULT_LANGUAGE
-    log_level = config.LOG_LEVEL_STR.lower()
+    lang = _safe_js_string(config.DEFAULT_LANGUAGE, _VALID_LANGS, "en")
+    log_level = _safe_js_string(config.LOG_LEVEL_STR.lower(), _VALID_LOG_LEVELS, "info")
     body = f'window.__DT_LANG = "{lang}";\nwindow.__DT_LOG_LEVEL = "{log_level}";'
     return Response(
         content=body,
@@ -183,10 +205,10 @@ def get_team_calendar_ical(
 ) -> Response:
     """Return ICS calendar with all duties (event_type duty only). Token validates user."""
     if not _is_valid_calendar_token(token):
-        return Response(status_code=404, content="Not found")
+        return JSONResponse(status_code=404, content={"detail": "Not found"})
     user = get_user_by_calendar_token(session, token)
     if user is None:
-        return Response(status_code=404, content="Not found")
+        return JSONResponse(status_code=404, content={"detail": "Not found"})
     cache_key = ("team_ics",)
     ics_bytes, found = ics_calendar_cache.get(cache_key)
     if not found:
@@ -224,10 +246,10 @@ def get_personal_calendar_ical(
     No Telegram auth; access is by secret token in the URL.
     """
     if not _is_valid_calendar_token(token):
-        return Response(status_code=404, content="Not found")
+        return JSONResponse(status_code=404, content={"detail": "Not found"})
     user = get_user_by_calendar_token(session, token)
     if user is None:
-        return Response(status_code=404, content="Not found")
+        return JSONResponse(status_code=404, content={"detail": "Not found"})
     cache_key = ("personal_ics", user.id)
     ics_bytes, found = ics_calendar_cache.get(cache_key)
     if not found: