feat(auth): replace email/password registration with OAuth2 (Google + GitHub)

- New /auth/[provider] route: generates state cookie, redirects to provider
- New /auth/[provider]/callback: exchanges code, fetches profile, auto-creates
  or links account, sets auth cookie
- pocketbase.ts: add oauth_provider/oauth_id to User; new getUserByOAuth(),
  createOAuthUser(), linkOAuthToUser() helpers; loginUser() drops email_verified gate
- pb-init-v3.sh: add oauth_provider + oauth_id fields (schema + migration)
- docker-compose.yml: GOOGLE/GITHUB client ID/secret env vars (replaces SMTP vars)
- Login page: two OAuth buttons (Google, GitHub) — register form removed
- /verify-email route and email.ts removed (provider handles email verification)
- /api/auth/register returns 410 (OAuth-only from now on)

scripts/pb-init-v3.sh

@@ -185,7 +185,9 @@ create "app_users" '{
     {"name":"email",                 "type":"text"},
     {"name":"email_verified",        "type":"bool"},
     {"name":"verification_token",    "type":"text"},
-    {"name":"verification_token_exp","type":"text"}
+    {"name":"verification_token_exp","type":"text"},
+    {"name":"oauth_provider",        "type":"text"},
+    {"name":"oauth_id",              "type":"text"}
   ]}'
 
 create "user_sessions" '{
@@ -254,5 +256,7 @@ add_field "app_users"      "email"                  "text"
 add_field "app_users"      "email_verified"         "bool"
 add_field "app_users"      "verification_token"     "text"
 add_field "app_users"      "verification_token_exp" "text"
+add_field "app_users"      "oauth_provider"         "text"
+add_field "app_users"      "oauth_id"               "text"
 
 log "done"