feat(i18n): add internationalization framework with 10 languages

- Create i18n utility module (app/utils/i18n.py) with translation loading,
  browser language detection, AI fallback, and l10n helpers
- Add JSON translation files for EN, DE, FR, ES, IT, PT, NL, PL, ZH, RU
- Add preferred_language column to UserProfile model with migration
- Register _() translation function as Jinja2 global
- Update base.html with translated navigation, footer, cookie notice
- Add language selector dropdown in nav bar (desktop + mobile)
- Create API endpoints for language preference (POST/GET /api/i18n/)
- Support language detection: user profile > cookie > Accept-Language > default

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

migrations/versions/027_add_user_language_preference.py

@@ -0,0 +1,28 @@
+"""Add preferred_language column to user_profiles for i18n support.
+
+Revision ID: 027_add_user_language_preference
+Revises: 026_add_scheduled_jobs
+Create Date: 2026-03-09
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "027_add_user_language_preference"
+down_revision: Union[str, None] = "026_add_scheduled_jobs"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Add preferred_language column to user_profiles table."""
+    op.add_column(
+        "user_profiles",
+        sa.Column("preferred_language", sa.String(10), nullable=True, server_default=None),
+    )
+
+
+def downgrade() -> None:
+    """Remove preferred_language column from user_profiles table."""
+    op.drop_column("user_profiles", "preferred_language")