feat(duplicates): add duplicate document detection and management

- Add near_duplicate_threshold config setting (default 0.85)
- New GET /api/duplicates endpoint listing all exact-duplicate groups
- New GET /api/files/{id}/duplicates endpoint returning exact + near-duplicates
- POST /api/ui-upload now returns immediate exact-duplicate warning (respects ENABLE_DEDUPLICATION)
- New /duplicates management UI with Exact Duplicates tab and Near-Duplicate Finder tab
- Add Duplicates link in admin nav menu (desktop + mobile)
- Document new config options in ConfigurationGuide.md and .env.demo
- 20 new tests covering all acceptance criteria

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -336,4 +336,14 @@ MEILISEARCH_URL=http://meilisearch:7700
 # Optional master/API key for secured Meilisearch instances
 # MEILISEARCH_API_KEY=your_master_key_here
 MEILISEARCH_INDEX_NAME=documents
-ENABLE_SEARCH=True
+ENABLE_SEARCH=True
+
+# **Duplicate Detection**
+# Exact duplicate detection (SHA-256) is always on during document processing.
+# The settings below control near-duplicate detection (same scanned content,
+# different hash) and the visibility of deduplication steps.
+ENABLE_DEDUPLICATION=True
+SHOW_DEDUPLICATION_STEP=True
+# Minimum cosine similarity score (0–1) for two documents to be flagged as
+# near-duplicates.  0.85 means 85 % semantic overlap.  Lower = more matches.
+NEAR_DUPLICATE_THRESHOLD=0.85