feat(translation): Post translation columns + settings + migration (#143 step 1)

Post gains post_title_translated / description_translated / translated_source_lang
/ translation_engine_version / translated_at — filled by the translate sweep so
viewing is instant. ImportSettings gains translation_enabled (OFF by default),
interpreter_base_url (EMPTY — no default host; the operator points it at their own
Interpreter proxy behind a reverse proxy) and translation_target_lang (en),
exposed + validated via /settings/import. Migration 0083. Settings defaults +
patch + validation test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/settings.py

@@ -32,6 +32,9 @@ _EDITABLE_FIELDS = (
     "extdl_mediafire_enabled",
     "extdl_dropbox_enabled",
     "extdl_pixeldrain_enabled",
+    "translation_enabled",
+    "interpreter_base_url",
+    "translation_target_lang",
 )
 
 # Per-host external-download toggles — all plain booleans, validated uniformly.
@@ -69,6 +72,9 @@ async def get_import_settings():
         "extdl_mediafire_enabled": row.extdl_mediafire_enabled,
         "extdl_dropbox_enabled": row.extdl_dropbox_enabled,
         "extdl_pixeldrain_enabled": row.extdl_pixeldrain_enabled,
+        "translation_enabled": row.translation_enabled,
+        "interpreter_base_url": row.interpreter_base_url,
+        "translation_target_lang": row.translation_target_lang,
     })
 
 
@@ -128,6 +134,15 @@ async def update_import_settings():
     for tog in _EXTDL_TOGGLE_FIELDS:
         if tog in body and not isinstance(body[tog], bool):
             return jsonify({"error": f"{tog} must be a boolean"}), 400
+    # Translation (#143): base URL may be empty (feature off until set — no
+    # default host; the operator points it at their own Interpreter proxy).
+    if "translation_enabled" in body and not isinstance(
+        body["translation_enabled"], bool
+    ):
+        return jsonify({"error": "translation_enabled must be a boolean"}), 400
+    for key in ("interpreter_base_url", "translation_target_lang"):
+        if key in body and not isinstance(body[key], str):
+            return jsonify({"error": f"{key} must be a string"}), 400
     if "series_suggest_threshold" in body:
         v = body["series_suggest_threshold"]
         if not isinstance(v, (int, float)) or isinstance(v, bool) or v < 0 or v > 1: