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/oauth.sql.go

@@ -0,0 +1,55 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.30.0
+// source: oauth.sql
+
+package db
+
+import (
+	"context"
+)
+
+const getOAuthProvider = `-- name: GetOAuthProvider :one
+SELECT provider, provider_id, user_id, email, created_at FROM oauth_providers
+WHERE provider = $1 AND provider_id = $2
+`
+
+type GetOAuthProviderParams struct {
+	Provider   string `json:"provider"`
+	ProviderID string `json:"provider_id"`
+}
+
+func (q *Queries) GetOAuthProvider(ctx context.Context, arg GetOAuthProviderParams) (OauthProvider, error) {
+	row := q.db.QueryRow(ctx, getOAuthProvider, arg.Provider, arg.ProviderID)
+	var i OauthProvider
+	err := row.Scan(
+		&i.Provider,
+		&i.ProviderID,
+		&i.UserID,
+		&i.Email,
+		&i.CreatedAt,
+	)
+	return i, err
+}
+
+const insertOAuthProvider = `-- name: InsertOAuthProvider :exec
+INSERT INTO oauth_providers (provider, provider_id, user_id, email)
+VALUES ($1, $2, $3, $4)
+`
+
+type InsertOAuthProviderParams struct {
+	Provider   string `json:"provider"`
+	ProviderID string `json:"provider_id"`
+	UserID     string `json:"user_id"`
+	Email      string `json:"email"`
+}
+
+func (q *Queries) InsertOAuthProvider(ctx context.Context, arg InsertOAuthProviderParams) error {
+	_, err := q.db.Exec(ctx, insertOAuthProvider,
+		arg.Provider,
+		arg.ProviderID,
+		arg.UserID,
+		arg.Email,
+	)
+	return err
+}