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>

migrations/versions/036_add_document_translation_fields.py

@@ -0,0 +1,44 @@
+"""Add document translation fields to files and user_profiles tables.
+
+Adds detected_language, default_language_text, and default_language_code to
+the files table so that a translated version of the document text can be
+stored alongside the original.
+
+Adds default_document_language to user_profiles so each user can override
+the system-wide default translation target language.
+
+Revision ID: 036_add_document_translation_fields
+Revises: 035_add_routing_rules
+Create Date: 2026-03-16
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "036_add_document_translation_fields"
+down_revision: Union[str, None] = "035_add_routing_rules"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add translation columns to files and user_profiles."""
+    with op.batch_alter_table("files") as batch_op:
+        batch_op.add_column(sa.Column("detected_language", sa.String(10), nullable=True))
+        batch_op.add_column(sa.Column("default_language_text", sa.Text(), nullable=True))
+        batch_op.add_column(sa.Column("default_language_code", sa.String(10), nullable=True))
+
+    with op.batch_alter_table("user_profiles") as batch_op:
+        batch_op.add_column(sa.Column("default_document_language", sa.String(10), nullable=True))
+
+
+def downgrade() -> None:
+    """Remove translation columns."""
+    with op.batch_alter_table("user_profiles") as batch_op:
+        batch_op.drop_column("default_document_language")
+
+    with op.batch_alter_table("files") as batch_op:
+        batch_op.drop_column("default_language_code")
+        batch_op.drop_column("default_language_text")
+        batch_op.drop_column("detected_language")