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/supporting_data.py

@@ -14,6 +14,12 @@ class SessionSupportingData(Base):
 
     id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     session_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("sessions.id", ondelete="CASCADE"), nullable=False, index=True)
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     label: Mapped[str] = mapped_column(String(255), nullable=False)
     data_type: Mapped[str] = mapped_column(Enum("text_snippet", "screenshot", name="supporting_data_type"), nullable=False)
     content: Mapped[str] = mapped_column(Text, nullable=False)