chore: archive v2.2 User Experience Polish milestone

Phases 28-31 archived to milestones/v2.2-phases/
Requirements and roadmap snapshots archived to milestones/

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.planning/milestones/v2.2-phases/29-image-presentation/29-04-SUMMARY.md

@@ -0,0 +1,44 @@
+---
+phase: 29
+plan: 04
+subsystem: backend
+tags: [migration, backfill, sharp]
+key-files:
+  created:
+    - scripts/backfill-dominant-colors.ts
+  modified:
+    - package.json
+metrics:
+  tasks: 2
+  commits: 1
+  files-changed: 2
+---
+
+# Plan 29-04 Summary: Backfill Migration Script
+
+## What was built
+- Created `scripts/backfill-dominant-colors.ts` backfill script
+- Processes items, globalItems (source URLs + image URLs), and threadCandidates
+- Extracts dominant color via Sharp 1x1 resize
+- Idempotent: skips records with existing dominantColor
+- Batch processing with 10 concurrent requests
+- Progress logging per table
+- Added `backfill:colors` npm script
+
+## Commits
+
+| Task | Commit | Description |
+|------|--------|-------------|
+| 1-2 | 6509b33 | Create backfill script and npm shortcut |
+
+## Deviations
+None.
+
+## Self-Check: PASSED
+- Script exists: YES
+- Processes all 3 tables: YES
+- Idempotent (isNull check): YES
+- Batch size 10: YES
+- Progress logging: YES
+- npm script exists: YES
+- Lint passes: YES