Fix high and medium security vulnerabilities

CSRF: upgrade session cookie sameSite from 'lax' to 'strict'.

Rate limiting: login endpoint now blocks an IP after 10 failed attempts
in a 15-minute window; resets on success. In-memory, no new dependency.

SESSION_SECRET: server exits at startup when NODE_ENV=production and
SESSION_SECRET is unset. docker-compose.yml updated to pass it via env;
.env.example added with generation instructions.

Security headers: add X-Content-Type-Options, X-Frame-Options, and
Referrer-Policy to all responses.

Sensitive data: routing_number and account_number are now omitted from
GET /api/account/:id responses for non-admin users.

Image size: logo upload capped at 512 KB in the account PUT handler.

Amount validation: checks (POST/PUT) and deposit items (POST/PUT) now
reject non-finite and non-positive amounts.

QBO import: uploaded file is rejected if its MIME type is not text or
a known CSV variant.

docker-compose.yml

@@ -12,6 +12,8 @@ services:
       - NODE_ENV=production
       - PORT=3000
       - DB_PATH=/app/data/check-printing.db
+      # Required in production — generate with: openssl rand -hex 32
+      - SESSION_SECRET=${SESSION_SECRET}
 
 volumes:
   check-printing-data: