feat: Phase 1 Group 1 — add account_id to core session tables

Migration sequence: add nullable → backfill via user_id/ai_session chain
→ verify zero NULLs → SET NOT NULL → CREATE INDEX.

Tables: sessions, attachments, session_supporting_data,
        session_resolution_outputs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/attachment.py

@@ -20,6 +20,12 @@ class Attachment(Base):
         ForeignKey("sessions.id"),
         nullable=False
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     node_id: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
     file_name: Mapped[str] = mapped_column(String(255), nullable=False)
     file_type: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)