fix(translate): exclude key columns from cache hash — only text + context fields

- _compute_source_hash now accepts context_keys parameter
- source_data is filtered to only include context-relevant fields (not row_id, primary keys)
- If no context_columns configured, hash is based on source_text + dict_hash + config_hash
- This ensures identical text in different rows produces a cache hit

backend/src/models/translate.py

@@ -133,7 +133,7 @@ class TranslationRecord(Base):
     token_count_output = Column(Integer, nullable=True)
     translation_duration_ms = Column(Integer, nullable=True)
     source_hash = Column(String, nullable=True,
-                         comment="SHA256(source_text+source_data+dict_snapshot_hash+config_hash) for cache dedup")
+                         comment="SHA256(source_text+context+dict_snapshot_hash+config_hash) for cache dedup — excludes key/identifier columns")
     created_at = Column(DateTime, default=lambda: datetime.now(UTC))
 
     languages = relationship("TranslationLanguage", back_populates="record")