feat(be): server-side deduplication for project & chat creation

[soumya813]

Summary

Prevent duplicate project/chat rows and incorrect token accounting when users click Generate multiple times or the client retries. Adds server-side deduplication (DB + app logic) and frontend calls the new backend API for authoritative project creation.
Problem

Rapid/retried client requests can create duplicate project and chat rows in the DB and cause incorrect token decrements. Client-only guards (localStorage or UI disable) are fragile against retries or network issues.

What I changed

Backend
Add SQLite-backed persistence + prompt normalization and md5 hash.
Add unique index on (user_id, prompt_hash) to prevent duplicate persisted rows.
Implement createOrGetProject(userId, prompt) which: checks for recent identical projects (2 minute window) and returns existing row or inserts a new one.
Add in-memory request coalescing (pendingRequests) to collapse near-simultaneous identical requests.
New/updated endpoints:
POST /project — createOrGetProject (request: { userId, prompt })
GET /projects/:userId
GET /chats/:userId
Chat rows are created server-side only for newly created projects to avoid duplicate chat events.
Improve error handling in existing endpoints.
Frontend
Add lightweight API client for backend endpoints and client-side request coalescing.
Update Home to call POST /project and to load projects/chats from server when signed in (falls back to localStorage).
Adds a migration helper to move localStorage data server-side (no destructive migration implemented).

Files changed

database.ts (new) — DB wrapper, prompt normalization, dedupe logic
index.ts (modified) — request coalescing, new endpoints, DB wiring
projects.db (new DB file)
projects.ts (new) — client API + coalescing
Home.tsx (modified) — uses backend API for project creation

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