feat(translation): add model, config, task, API, and UI for default document language translation

- Add detected_language, default_language_text, default_language_code columns to FileRecord
- Add default_document_language column to UserProfile
- Add DEFAULT_DOCUMENT_LANGUAGE config setting (defaults to "en")
- Create translate_to_default_language Celery task
- Integrate translation trigger into embed_metadata_into_pdf pipeline
- Add /api/files/{id}/translate and /api/files/{id}/translation/default API endpoints
- Add /files/{id}/text/default-language view endpoint
- Update file_view.html with translation sections (default language, on-the-fly)
- Add translation keys to en.json
- Create Alembic migration 036
- Update .env.demo

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/config.py

@@ -166,6 +166,25 @@ class Settings(BaseSettings):
     google_docai_location: str = "us"  # Processor location, e.g. "us" or "eu"
     external_hostname: str = "localhost"  # Default to localhost
 
+    # ---------------------------------------------------------------------------
+    # Document Translation Settings
+    # ---------------------------------------------------------------------------
+    # Default target language for automatic document translation (ISO 639-1 code).
+    # After OCR / metadata extraction, if the detected document language differs
+    # from this value the system translates the extracted text into this language
+    # and stores it alongside the original.  Other language translations are
+    # generated on the fly via the AI provider and are NOT persisted.
+    # Per-user overrides are stored in UserProfile.default_document_language.
+    default_document_language: str = Field(
+        default="en",
+        description=(
+            "ISO 639-1 language code for the default translation target "
+            "(e.g. 'en', 'de', 'fr'). Documents whose detected language "
+            "differs are automatically translated into this language after "
+            "processing. Default: 'en' (English)."
+        ),
+    )
+
     # Authentication settings
     auth_enabled: bool = True  # Default to enabled
     admin_username: Optional[str] = None