docs(project): document DUPLICATE_HASH_THRESHOLD in .env.example

The dedup pairs rebuild reads this tunable (default 10/64); it was added to the
backend config but never documented for operators. No other new env vars were
introduced by duplicate detection — the dedup compose service reuses the
existing PUID/PGID/FILES_DIR/THUMBS_DIR.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -125,6 +125,16 @@ THUMB_CONCURRENCY=0
 # ---------------------------------------------------------------------------
 IMPORT_PATH=/data/import
 
+# ---------------------------------------------------------------------------
+# Duplicate detection
+# ---------------------------------------------------------------------------
+# 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
+
 # ---------------------------------------------------------------------------
 # Static SPA
 # ---------------------------------------------------------------------------