fix: include tags, created_at, proof_count in graph table_rows

[cdbartholomew]

Summary

• The graph endpoint's table_rows response was missing three fields that the control plane UI expects:
  • tags: memory unit tags (shown in Tags column for all fact types)
  • created_at: creation timestamp (shown in Created column for mental models)
  • proof_count: source memory count (shown in Sources column for mental models)
• All three columns exist on the memory_units table but were not being selected in the SQL query or included in the response dict.
• Without this fix, the Tags column shows -, Created shows -, and Sources always shows 1 (due to || 1 fallback in the UI).

Test plan

• Load the Graph/Table view for a bank with tagged memories
• Verify Tags column shows actual tags instead of -
• Switch to mental models view and verify Sources shows real proof_count
• Verify Created column shows actual dates for mental models