fix(security): add helmet and express-mongo-sanitize global middleware

[sricharan12-hub]

Overview

Adds centralized defense-in-depth middleware to backend/src/server.js. The app
previously relied solely on per-handler validation to stay safe from NoSQL
(operator) injection, with no global safety net and no HTTP security headers.

Closes #79

Changes

• helmet() — registered early in the middleware chain to set secure HTTP
  response headers (X-Content-Type-Options: nosniff, X-Frame-Options, HSTS, etc.).
• express-mongo-sanitize() — registered after body parsing to strip keys
  containing $ or . from req.body, req.params, and req.query, so an
  operator-shaped payload can never reach a Mongo query as an operator object —
  regardless of whether the individual handler validates.

Both run before the routes. The change is purely additive: mongo-sanitize
only removes $/.-prefixed keys, which legitimate auth and notes payloads never
use, so existing behavior is unchanged.

Express here is v4, so the known express-mongo-sanitize read-only req.query
crash (Express 5) does not apply.

Acceptance criteria

• helmet and express-mongo-sanitize added and registered before routes.
• A request body such as { "email": { "$gt": "" } } is sanitized and cannot reach a query as an operator object.
• Existing functionality (auth + notes CRUD) continues to work unchanged.

Testing

Ran the exact middleware chain (helmet → express.json → express-mongo-sanitize)
against a live request:

• Body { email: { $gt: "" }, name: "ok", nested: { "$where": "...", safe: 1 } }
  → { email: {}, name: "ok", nested: { safe: 1 } } — operator keys stripped at
  every level, legitimate fields preserved.
• Query ?filter[$gt]= → { filter: {} }.
• Response carried x-content-type-options: nosniff and x-frame-options: SAMEORIGIN
  from helmet.

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Reviewing files that changed from the base of the PR and between 87148ba and 6b19bcc.

⛔ Files ignored due to path filters (1)

• backend/package-lock.json is excluded by !**/package-lock.json

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• backend/package.json
• backend/src/server.js

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