feat(translate): multi-language optimization (Phase 11)

- Auto-detection of source language per row via LLM (US6)
- Multi-target translation — one LLM call for N languages (US1-US3)
- Language-aware storage: TranslationLanguage, per-language stats
- Multilingual dictionaries with language-pair-aware filtering (US7)
- Inline correction on any run result + submit-to-dictionary (US8)
- Context-aware dictionary: auto-capture row context, usage notes,
  Jaccard similarity, priority flagging in LLM prompts (US8b)
- Configurable preview sample size 1-100, cost warning at >30
- Per-language history & metrics with MetricSnapshot preservation
- 36 files, +5022/-373, all specs GRACE-Poly v2.6 compliant

backend/alembic/script.py.mako

@@ -0,0 +1,28 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}