Add OAuth2/OpenID Connect authorization server

Implement a full OAuth2 2.0 and OpenID Connect 1.0 authorization
server with support for authorization code flow (with PKCE),
refresh token rotation, device authorization grant, dynamic
client registration, token introspection, and token revocation.

Includes database schema, coredata layer, service logic, HTTP
handlers, OIDC discovery endpoint, and JWKS publishing.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/net/net.go

@@ -0,0 +1,30 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package net
+
+import "net"
+
+// IsLoopback reports whether host is a loopback address. It recognizes
+// "localhost" by name and delegates to net.IP.IsLoopback for IP addresses,
+// which covers 127.0.0.0/8, ::1, and IPv4-mapped variants like
+// ::ffff:127.0.0.1.
+func IsLoopback(host string) bool {
+	if host == "localhost" {
+		return true
+	}
+
+	ip := net.ParseIP(host)
+	return ip != nil && ip.IsLoopback()
+}

pkg/net/net_test.go

@@ -0,0 +1,84 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package net_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.probo.inc/probo/pkg/net"
+)
+
+func TestIsLoopback(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		host string
+		want bool
+	}{
+		// Localhost by name.
+		{"localhost", "localhost", true},
+
+		// IPv4 loopback addresses (127.0.0.0/8).
+		{"ipv4 canonical loopback", "127.0.0.1", true},
+		{"ipv4 loopback high octet", "127.255.255.255", true},
+		{"ipv4 loopback alternate", "127.0.0.2", true},
+		{"ipv4 loopback 127.1.2.3", "127.1.2.3", true},
+
+		// IPv6 loopback.
+		{"ipv6 loopback", "::1", true},
+
+		// IPv4-mapped IPv6 loopback.
+		{"ipv4-mapped ipv6 loopback", "::ffff:127.0.0.1", true},
+		{"ipv4-mapped ipv6 loopback alternate", "::ffff:127.0.0.2", true},
+
+		// Non-loopback addresses.
+		{"ipv4 private 10.x", "10.0.0.1", false},
+		{"ipv4 private 192.168.x", "192.168.1.1", false},
+		{"ipv4 private 172.16.x", "172.16.0.1", false},
+		{"ipv4 public", "8.8.8.8", false},
+		{"ipv4 all interfaces", "0.0.0.0", false},
+		{"ipv6 all interfaces", "::", false},
+		{"ipv6 link-local", "fe80::1", false},
+		{"ipv6 public", "2001:db8::1", false},
+		{"ipv4-mapped ipv6 non-loopback", "::ffff:192.168.1.1", false},
+
+		// Hostnames that are not localhost.
+		{"example.com", "example.com", false},
+		{"localhost.localdomain", "localhost.localdomain", false},
+		{"myhost", "myhost", false},
+
+		// Edge cases.
+		{"empty string", "", false},
+		{"whitespace", " ", false},
+		{"localhost with trailing dot", "localhost.", false},
+		{"uppercase LOCALHOST", "LOCALHOST", false},
+		{"mixed case Localhost", "Localhost", false},
+		{"128.0.0.1 not loopback", "128.0.0.1", false},
+		{"126.255.255.255 not loopback", "126.255.255.255", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(
+			tt.name,
+			func(t *testing.T) {
+				t.Parallel()
+
+				assert.Equal(t, tt.want, net.IsLoopback(tt.host))
+			},
+		)
+	}
+}