feat(thumbnails): generate cached previews for Drive files

Cargo.toml

@@ -20,6 +20,7 @@ pdfs-fuse = { path = "crates/pdfs-fuse" }
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "fs", "io-util", "time", "signal"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+base64 = "0.22"
 thiserror = "2"
 anyhow = "1"
 # Non-poisoning mutexes. A handler that panics while holding a lock must not