feat(control-plane): add Constellation view with Pretext canvas rendering

[nicoloboschi]

Summary

• New Constellation view as the default memory visualization in the control plane, powered by @chenglou/pretext for DOM-free text layout on canvas
• Canvas-rendered zoomable/pannable memory map with spatial label deconfliction — handles 1000+ nodes at 60fps
• Nodes colored by link-count heat gradient (brand teal → cyan → blue), with star-like varied size/opacity
• Hover shows rich tooltip with full memory metadata (text, entities, tags, dates, document ID, proof count)
• Hover highlights connected nodes and links, dims unrelated ones
• Click to select nodes and view details in the side panel
• Fullscreen mode toggle (Esc to exit)
• Graph API performance fix: entity query was doing a full table scan (missing WHERE clause), links query was returning 500k+ uncapped edges with an expensive DISTINCT ON sort — now filters by visible units and caps at 10k edges sorted by weight. ~10x faster (8s → 0.8s for 1000 nodes).

Test plan

• Open control plane, navigate to a bank with memories
• Verify Constellation is the default view and renders nodes/links
• Scroll to zoom, drag to pan, hover to see tooltips and link highlights
• Click a node to open the detail panel
• Test fullscreen toggle
• Switch to Graph/Table/Timeline views and back — verify they still work
• Test with Observations tab — verify observation link inheritance still works
• Verify graph API response time is fast (~1s for 1000 nodes)