fix: filter near-uniform pHashes, lower threshold to 3

Near-uniform images (sand close-ups, solid-color shots, blank frames)
have almost no gradient transitions so their dHash has very few set bits.
These match many unrelated photos because "low contrast ≈ low contrast."

- Skip photos whose pHash has <8 or >56 set bits out of 64 before
  comparing — they can't be reliably compared by dHash
- Lower default threshold 5 → 3 (≤3 differing bits = genuinely near-
  identical, e.g. same photo saved at different compression levels)
- Report "N compared" in scan result so skipped photos are visible

Run "Scan library" to replace existing pairs with clean results.

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:  '5',
+  dedupe_threshold:  '3',
   // iCloud Photos sync
   icloud_enabled:    '0',
   icloud_apple_id:   '',