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/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()
     )