feat: depot API

torrential/docs/protocol.md

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+# Protocol
+
+`torrential` implements the Depot API as defined by https://developer.droposs.org/web/depot, and it prefixed with `/api/v1/depot` for NGINX proxying.
+
+It also has the following endpoints, only accessible by the Drop server for security reasons:
+ - `/key` for sharing the authentication key from the Drop server to torrential
+ - `/invalidate` for pre-emptivel clearing the download context cache. Contexts are automatically cleared regardless, so this endpoint failing is not a hard error on the Drop side
+ - `/healthcheck`. Does healthcheck.

torrential/docs/structure.md

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+# Structure
+Torrential is a typical Rust project. Source files are in `src/`. 
+
+`handlers.rs` contains most non-download endpoint handlers. `serve.rs` contains the download endpoint handler.
+
+`remote.rs` handles communciating with the Drop server.