feat: add default_language to site config for banner i18n

Site owners can now set a default language for the consent banner,
overriding browser auto-detection. When null the banner uses
navigator.language / data-locale as before (existing behaviour).

Changes:
- DB: add default_language column to site_configs (nullable, String(10))
- API model: SiteConfig.default_language field
- API schema: SiteConfigCreate/Update/Response schemas
- Config resolver: pass default_language through to public config
- Banner types: SiteConfig.default_language field
- Banner logic: use config.default_language ?? detectLocale()
- Admin UI: SiteConfigTab dropdown with auto-detect option
- Admin UI types: SiteConfig.default_language added

apps/api/alembic/versions/0004_default_language.py

@@ -0,0 +1,34 @@
+"""default_language on site configs
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-06-15
+
+Allows site owners to set a default language for the banner,
+overriding the browser's auto-detection. The banner script uses
+this when loading translations.
+
+The column is nullable — NULL means "auto-detect from browser".
+"""
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision: str = "0004"
+down_revision: str | Sequence[str] | None = "0003"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "site_configs",
+        sa.Column("default_language", sa.String(10), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("site_configs", "default_language")