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/src/core/config_models.py

@@ -141,6 +141,26 @@ class GlobalSettings(BaseModel):
     # Global worker limit for concurrent validation runs
     GLOBAL_VALIDATION_WORKER_LIMIT: int = 3
 
+    # Allowed languages for translation (BCP-47 codes)
+    allowed_languages: list[str] = Field(
+        default_factory=lambda: [
+            "ru", "en", "de", "fr", "es", "it", "pt", "zh", "ja", "ko",
+            "ar", "tr", "nl", "pl", "sv", "da", "fi", "cs", "hu", "ro",
+            "vi", "th", "he", "id", "ms",
+        ]
+    )
+
+    @field_validator("allowed_languages")
+    @classmethod
+    def validate_allowed_languages(cls, v: list[str]) -> list[str]:
+        import re
+        for tag in v:
+            if not tag or not tag.strip():
+                raise ValueError("Empty language code is not allowed")
+            if not re.match(r'^[a-zA-Z]{2,8}(-[a-zA-Z0-9]{1,8})*$', tag.strip()):
+                raise ValueError(f"Invalid BCP-47 language code: {tag}")
+        return v
+
 
 # #endregion GlobalSettings