Enhance message_read handling: monotonic read cursor, privacy-aware r…

[martinshub-tech]

this pr closes #186

Changes Overview
apps/backend/src/socket/messaging.ts

Imported the users table.
Implemented monotonic read cursor logic to prevent the read cursor from moving backwards.
Added per‑user privacy setting (sendReadReceipts) to conditionally broadcast read_receipt events.
Safe user lookup with fallback handling.
apps/backend/src/db/schema.ts

Added sendReadReceipts boolean column to the users table (default true).
Test environment fallbacks

apps/backend/src/lib/jwt.ts – provides a default secret (test‑secret) and ensures process.env.JWT_SECRET is set for tests.
apps/backend/src/db/index.ts – supplies a fallback PostgreSQL connection string for test runs.
package.json

Added missing dev‑dependencies:
supertest
express
jsonwebtoken
ioredis
@stellar/stellar-sdk
vitest
Updated imports and added necessary safeguards.

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