security: remove SHA-256 password fallback, CSPRNG for verification codes and keys

[NetworkTheoryAppliedResearchInstitute]

Fixes the credential-strength findings from CodeQL (alerts #10–#12):

1. backend/utils/bcrypt.js — the module silently fell back to unsalted, single-round SHA-256 for password hashing whenever bcryptjs/bcrypt failed to load. bcryptjs is a declared dependency, so the fallback only ever fired on a broken install — which should fail loudly, not quietly store crackable hashes. It now throws with a clear message instead. No hash-format migration needed: any correctly-installed deployment was already using bcryptjs.
2. backend/controllers/user_controller.js — 6-digit account verification codes were generated with Math.random(), which is predictable — a guessable code lets an attacker verify an account registered against someone else's email/phone. Now crypto.randomInt(100000, 1000000).
3. backend/controllers/key_controller.js — generateMcElieseKey() produced ~13 chars of Math.random().toString(36); now 32 bytes of crypto.randomBytes (base64url). Note: despite the name, this is a random token generator, not an actual McEliece implementation — if real post-quantum keys are intended, that's a separate work item.

All three files pass node --check. Remaining open CodeQL findings (path injection in models/message.js, missing rate limiting on eight routes, workflow permissions) are deliberately not in this PR — happy to follow up separately.

🤖 Generated with Claude Code

[csecrestjr]

Update README.es.md:94:node server.js with this

starts the server (e.g., http://localhost:5000)