chore: Initial database scaffolding (#2)

* chore: Initial database scaffolding

This implements migrations and code generation for interfacing with a PostgreSQL database.

A dependency is added for the "postgres" binary on the host, but that seems like an acceptable requirement considering it's our primary database.

An in-memory database object can be created for simple cross-OS and fast testing.

* Run tests in CI

* Use Docker instead of binaries on the host

* Skip database tests on non-Linux operating systems

* chore: Add golangci-lint and codecov

* Use consistent file names

database/querier.go

@@ -0,0 +1,13 @@
+// Code generated by sqlc. DO NOT EDIT.
+
+package database
+
+import (
+	"context"
+)
+
+type querier interface {
+	ExampleQuery(ctx context.Context) error
+}
+
+var _ querier = (*sqlQuerier)(nil)