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/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."""