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>

app/config.py

@@ -316,6 +316,13 @@ class Settings(BaseSettings):
             " If False, the check is still performed but not displayed. Default: True."
         ),
     )
+    near_duplicate_threshold: float = Field(
+        default=0.85,
+        description=(
+            "Minimum cosine similarity score (0–1) between two documents' text embeddings to consider "
+            "them near-duplicates.  Higher values require closer content matches.  Default: 0.85."
+        ),
+    )
 
     # Text quality check - AI-based assessment of embedded PDF text
     enable_text_quality_check: bool = Field(