Reorganized API + added db migrations

Read the README file for more informations

backend/README.md

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+# Prerequisites for development
+
+-   Installing dependencies: `go mod tidy`
+-   Verifying that the `.env` file is present.
+
+## Running migrations
+
+A migration cli is made available through the package: `./cmd/migrate`
+At first it is need to run: `go run ./cmd/migrate db init` to initialize the
+tables needed by golang's bun.
+
+Then, if there migrations needed to be applied, run: `go run ./cmd/migrate db migrate`
+
+Everything should be taken care of.
+
+## Running the server
+
+The server can be ran as easily as: `go run .`