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/session_resolution_output.py

@@ -23,6 +23,12 @@ class SessionResolutionOutput(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("ai_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,
+    )
     output_type: Mapped[str] = mapped_column(String(30), nullable=False)
     generated_content: Mapped[str] = mapped_column(Text, nullable=False)
     structured_data: Mapped[Optional[dict[str, Any]]] = mapped_column(JSONB, nullable=True, comment="For KB: {symptoms, root_cause, steps, tags}")