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:

backend/app/models/import_settings.py

@@ -92,6 +92,21 @@ class ImportSettings(Base):
         Boolean, nullable=False, default=True, server_default="true",
     )
 
+    # -- Post-text translation via the Interpreter LAN service (milestone 143).
+    # Off by default with NO default host — it needs a reachable Interpreter
+    # service (the operator's, behind a reverse proxy), which not every install
+    # has; the operator sets the URL and flips it on. Empty base_url OR disabled
+    # → the translate sweep no-ops.
+    translation_enabled: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default="false",
+    )
+    interpreter_base_url: Mapped[str] = mapped_column(
+        Text, nullable=False, default="", server_default="",
+    )
+    translation_target_lang: Mapped[str] = mapped_column(
+        Text, nullable=False, default="en", server_default="en",
+    )
+
     @classmethod
     async def load(cls, session) -> ImportSettings:
         """The singleton settings row (id=1), via an async session."""

backend/app/models/post.py

@@ -47,6 +47,24 @@ class Post(Base):
     description: Mapped[str | None] = mapped_column(Text, nullable=True)
     attachment_count: Mapped[int | None] = mapped_column(Integer, nullable=True)
 
+    # -- Post-text translation (milestone 143). Filled by the translate_posts
+    # sweep via the Interpreter LAN service so viewing is instant.
+    # translated_source_lang is the DETECTED original language; "en" (or a
+    # passthrough) means nothing to translate and the *_translated columns stay
+    # NULL. engine_version keys the Interpreter cache — re-runs are ~1ms and a
+    # model upgrade re-translates instead of serving stale.
+    post_title_translated: Mapped[str | None] = mapped_column(Text, nullable=True)
+    description_translated: Mapped[str | None] = mapped_column(Text, nullable=True)
+    translated_source_lang: Mapped[str | None] = mapped_column(
+        String(8), nullable=True
+    )
+    translation_engine_version: Mapped[str | None] = mapped_column(
+        String(128), nullable=True
+    )
+    translated_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+
     downloaded_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )