Fix auth redirect to 0.0.0.0 and credentials sign-in error

- Add AUTH_URL env var to docker-compose.yml and .env/.env.example so
  NextAuth builds correct redirect URLs instead of falling back to the
  Docker bind address (0.0.0.0:3000)
- Normalize email to lowercase in authorize() to match how the register
  route stores emails, preventing case-sensitive lookup mismatches

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/auth.ts

@@ -21,7 +21,7 @@ export const { handlers, auth, signIn, signOut } = NextAuth({
         password: { label: "Password", type: "password" },
       },
       async authorize(credentials) {
-        const email = credentials?.email as string | undefined;
+        const email = (credentials?.email as string | undefined)?.trim().toLowerCase();
         const password = credentials?.password as string | undefined;
 
         if (!email || !password) {