chore(project): lower duplicate hash threshold to 4 in example env

The distance histogram over a real ~65k-pair table is flat-to-rising from
distance 1 and nearly doubles at 10 — the 8-10 band is dominated by
coincidental matches, not duplicates. Default the example to 4 (the valley
minimum) to keep the genuine-duplicate signal and cut the pairs to cluster.

.env.example

@@ -131,9 +131,12 @@ IMPORT_PATH=/data/import
 # Maximum perceptual-hash distance (Hamming, out of 64 bits) for two files to be
 # treated as duplicate candidates. Lower = stricter (fewer, more confident
 # matches); higher = looser (catches more re-encodes/resizes but risks false
-# positives). Used only by the dedup tool's pairs rebuild — see the dedup CLI /
-# `docker compose run --rm dedup`. Default 10.
-DUPLICATE_HASH_THRESHOLD=10
+# positives). On real libraries the distance histogram climbs steeply in the 8–10
+# band — coincidental "vaguely similar" pairs, not duplicates — so 4 keeps the
+# genuine-duplicate signal without that noise (and far fewer pairs to cluster).
+# Used only by the dedup tool's pairs rebuild — see the dedup CLI /
+# `docker compose run --rm dedup`. Code default is 10.
+DUPLICATE_HASH_THRESHOLD=4
 
 # ---------------------------------------------------------------------------
 # Static SPA