Initial implementation of duplicate finder

Full project per spec: FastAPI backend, 4-method duplicate detection
(SHA-256, phash, EXIF, filesize), Google Takeout pre-processor,
4 scan modes, and dark-theme vanilla JS gallery frontend.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements.txt

@@ -0,0 +1,7 @@
+fastapi==0.115.6
+uvicorn==0.32.1
+Pillow==11.0.0
+imagehash==4.3.1
+pillow-heif==0.21.0
+jinja2==3.1.4
+aiofiles==24.1.0