fix: eliminate false-positive duplicate mega-groups

The union-find algorithm caused transitivity chains: A≈B and B≈C merged
A, B, C into one group even if A and C were completely different photos,
producing the 5000+ "duplicate" groups that were wrong.

- getGroups now returns individual pairs (2 photos each) — what was
  actually flagged by the pHash comparison, no chaining
- scan() clears all existing pairs on each run so the result exactly
  matches the current threshold (no stale loose pairs from prior scans)
- Default dedupe_threshold lowered from 10 → 5 (fewer false positives;
  distance ≤5 means the images are genuinely near-identical)

Run "Scan library" once to replace the old results with clean pairs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

worker/db.js

@@ -7,7 +7,7 @@ const SETTING_DEFAULTS = {
   thumb_quality:     '80',
   preview_size:      '1600',
   preview_quality:   '85',
-  dedupe_threshold:  '10',
+  dedupe_threshold:  '5',
   // iCloud Photos sync
   icloud_enabled:    '0',
   icloud_apple_id:   '',