feat(server): add POST /api/auth/register and /api/auth/login

- Add register endpoint with email/password validation, bcrypt hashing
- Add login endpoint with rate limiting per IP (5 attempts/min)
- Add helper functions: validatePassword, hashPassword, comparePassword, isAdminEmail
- Add checkLoginRateLimit for brute-force protection
- Add bcrypt dependency
- Remove avatarType column from User (migration)

server/prisma/migrations/20260522062112_remove_avatar_type/migration.sql

@@ -0,0 +1,28 @@
+/*
+  Warnings:
+
+  - You are about to drop the column `avatarType` on the `User` table. All the data in the column will be lost.
+
+*/
+-- RedefineTables
+PRAGMA defer_foreign_keys=ON;
+PRAGMA foreign_keys=OFF;
+CREATE TABLE "new_User" (
+    "id" TEXT NOT NULL PRIMARY KEY,
+    "email" TEXT NOT NULL,
+    "displayName" TEXT,
+    "firstName" TEXT,
+    "lastName" TEXT,
+    "gender" TEXT,
+    "avatar" TEXT,
+    "avatarStyle" TEXT,
+    "passwordHash" TEXT,
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" DATETIME NOT NULL
+);
+INSERT INTO "new_User" ("avatar", "avatarStyle", "createdAt", "displayName", "email", "firstName", "gender", "id", "lastName", "passwordHash", "updatedAt") SELECT "avatar", "avatarStyle", "createdAt", "displayName", "email", "firstName", "gender", "id", "lastName", "passwordHash", "updatedAt" FROM "User";
+DROP TABLE "User";
+ALTER TABLE "new_User" RENAME TO "User";
+CREATE UNIQUE INDEX "User_email_key" ON "User"("email");
+PRAGMA foreign_keys=ON;
+PRAGMA defer_foreign_keys=OFF;