Add GitHub OAuth login with provider registry

Implement OAuth 2.0 login via GitHub as an alternative to email/password.
Uses a provider registry pattern (internal/auth/oauth/) so adding Google
or other providers later requires only a new Provider implementation.

Flow: GET /v1/auth/oauth/github redirects to GitHub, callback exchanges
the code for a user profile, upserts the user + team atomically, and
redirects to the frontend with a JWT token.

Key changes:
- Migration: make password_hash nullable, add oauth_providers table
- Provider registry with GitHubProvider (profile + email fallback)
- CSRF state cookie with HMAC-SHA256 validation
- Race-safe registration (23505 collision retries as login)
- Startup validation: CP_PUBLIC_URL required when OAuth is configured

Not fully tested — needs integration tests with a real GitHub OAuth app
and end-to-end testing with the frontend callback page.

internal/db/users.sql.go

@@ -7,6 +7,8 @@ package db
 
 import (
 	"context"
+
+	"github.com/jackc/pgx/v5/pgtype"
 )
 
 const getUserByEmail = `-- name: GetUserByEmail :one
@@ -50,9 +52,9 @@ RETURNING id, email, password_hash, created_at, updated_at
 `
 
 type InsertUserParams struct {
-	ID           string `json:"id"`
-	Email        string `json:"email"`
-	PasswordHash string `json:"password_hash"`
+	ID           string      `json:"id"`
+	Email        string      `json:"email"`
+	PasswordHash pgtype.Text `json:"password_hash"`
 }
 
 func (q *Queries) InsertUser(ctx context.Context, arg InsertUserParams) (User, error) {
@@ -67,3 +69,27 @@ func (q *Queries) InsertUser(ctx context.Context, arg InsertUserParams) (User, e
 	)
 	return i, err
 }
+
+const insertUserOAuth = `-- name: InsertUserOAuth :one
+INSERT INTO users (id, email)
+VALUES ($1, $2)
+RETURNING id, email, password_hash, created_at, updated_at
+`
+
+type InsertUserOAuthParams struct {
+	ID    string `json:"id"`
+	Email string `json:"email"`
+}
+
+func (q *Queries) InsertUserOAuth(ctx context.Context, arg InsertUserOAuthParams) (User, error) {
+	row := q.db.QueryRow(ctx, insertUserOAuth, arg.ID, arg.Email)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Email,
+		&i.PasswordHash,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+	)
+	return i, err
+}