feat: add media upload pipeline with multipart support, blurhash generation, and R2 storage

- Add project_media table with image/video variants, ordering, and metadata
- Implement multipart upload handlers with 50MB limit
- Generate blurhash placeholders and resize images to thumb/medium/full variants
- Update ProjectCard to use media carousel instead of mock gradients
- Add MediaManager component for drag-drop upload and reordering

Cargo.toml

@@ -11,12 +11,14 @@ path = "src/main.rs"
 argon2 = "0.5"
 aws-config = "1.8.12"
 aws-sdk-s3 = "1.119.0"
-axum = "0.8.8"
+axum = { version = "0.8.8", features = ["multipart"] }
 axum-extra = { version = "0.12", features = ["cookie"] }
+blurhash = "0.2.3"
 clap = { version = "4.5.54", features = ["derive", "env"] }
 dashmap = "6.1.0"
 dotenvy = "0.15"
 futures = "0.3.31"
+image = "0.25.9"
 include_dir = "0.7.4"
 mime_guess = "2.0.5"
 moka = { version = "0.12.12", features = ["future"] }