Add beacon.watch (#3362)

> This is currently a WIP and all features are subject to alteration or removal at any time.

## Overview

The successor to #2873.

Contains the backbone of `beacon.watch` including syncing code, the initial API, and several core database tables.

See `watch/README.md` for more information, requirements and usage.

watch/src/server/error.rs

@@ -0,0 +1,50 @@
+use crate::database::Error as DbError;
+use axum::Error as AxumError;
+use axum::{http::StatusCode, response::IntoResponse, Json};
+use hyper::Error as HyperError;
+use serde_json::json;
+
+#[derive(Debug)]
+pub enum Error {
+    Axum(AxumError),
+    Hyper(HyperError),
+    Database(DbError),
+    BadRequest,
+    NotFound,
+    Other(String),
+}
+
+impl IntoResponse for Error {
+    fn into_response(self) -> axum::response::Response {
+        let (status, error_message) = match self {
+            Self::BadRequest => (StatusCode::BAD_REQUEST, "Bad Request"),
+            Self::NotFound => (StatusCode::NOT_FOUND, "Not Found"),
+            _ => (StatusCode::INTERNAL_SERVER_ERROR, "Internal Server Error"),
+        };
+        (status, Json(json!({ "error": error_message }))).into_response()
+    }
+}
+
+impl From<HyperError> for Error {
+    fn from(e: HyperError) -> Self {
+        Error::Hyper(e)
+    }
+}
+
+impl From<AxumError> for Error {
+    fn from(e: AxumError) -> Self {
+        Error::Axum(e)
+    }
+}
+
+impl From<DbError> for Error {
+    fn from(e: DbError) -> Self {
+        Error::Database(e)
+    }
+}
+
+impl From<String> for Error {
+    fn from(e: String) -> Self {
+        Error::Other(e)
+    }
+}