feat(backend): drop dictionary dialect columns, add allowed_languages

- Removes source_dialect and target_dialect from TerminologyDictionary model,
  schemas, routes, helper serialization, and all tests
- Adds allowed_languages config field (BCP-47 language codes) to GlobalSettings
  with validation, consolidated settings response, and API endpoint
- Adds alembic migration f1a2b3c4d5e6 to drop the two columns

backend/alembic/versions/f1a2b3c4d5e6_drop_dict_dialect_columns.py

@@ -0,0 +1,44 @@
+"""drop source_dialect/target_dialect from terminology_dictionaries
+
+Revision ID: f1a2b3c4d5e6
+Revises: dabc97097e0e
+Create Date: 2026-06-02 12:00:00.000000
+
+"""
+from collections.abc import Sequence
+
+from alembic import op
+from sqlalchemy import inspect
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f1a2b3c4d5e6'
+down_revision: str | Sequence[str] | None = 'dabc97097e0e'
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def _column_exists(table_name: str, column_name: str) -> bool:
+    conn = op.get_bind()
+    inspector = inspect(conn)
+    columns = [c["name"] for c in inspector.get_columns(table_name)]
+    return column_name in columns
+
+
+def upgrade() -> None:
+    """Drop source_dialect and target_dialect from terminology_dictionaries."""
+    if not _column_exists("terminology_dictionaries", "source_dialect"):
+        return
+    op.drop_column("terminology_dictionaries", "source_dialect")
+    op.drop_column("terminology_dictionaries", "target_dialect")
+
+
+def downgrade() -> None:
+    """Re-add source_dialect and target_dialect to terminology_dictionaries."""
+    import sqlalchemy as sa
+    op.add_column("terminology_dictionaries", sa.Column(
+        "source_dialect", sa.String(), nullable=False, server_default="",
+    ))
+    op.add_column("terminology_dictionaries", sa.Column(
+        "target_dialect", sa.String(), nullable=False, server_default="",
+    ))