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/models.py

@@ -84,6 +84,19 @@ class FileRecord(Base):
     # Processing pipeline assigned to this file (NULL = use system default)
     pipeline_id = Column(Integer, ForeignKey(_PIPELINES_ID_FK), nullable=True, index=True)
 
+    # Detected document language (ISO 639-1 code, e.g. "de", "en", "fr")
+    # Extracted from AI metadata during processing; cached here for fast access.
+    detected_language = Column(String(10), nullable=True)
+
+    # Default-language translation of the extracted text.
+    # Stored when the detected language differs from the user's/system default
+    # document language.  Only the original text and this translation are persisted;
+    # other languages are translated on the fly via the AI provider.
+    default_language_text = Column(Text, nullable=True)
+
+    # ISO 639-1 code of the default-language translation stored above (e.g. "en").
+    default_language_code = Column(String(10), nullable=True)
+
     # Timestamp when we inserted this record
     created_at = Column(DateTime(timezone=True), server_default=func.now(), index=True)
 
@@ -282,6 +295,12 @@ class UserProfile(Base):
     # NULL means "auto-detect from browser Accept-Language header"
     preferred_language = Column(String(10), nullable=True)
 
+    # Default document language for translated versions (ISO 639-1 code).
+    # When a document's detected language differs from this value, the system
+    # automatically generates and stores a translation into this language.
+    # NULL means "use the global DEFAULT_DOCUMENT_LANGUAGE setting".
+    default_document_language = Column(String(10), nullable=True)
+
     # UI colour scheme preference: "light" | "dark" | "system" (NULL = "system")
     preferred_theme = Column(String(10), nullable=True)