refactor: replace uuid with int64 for user and todo IDs

[Gautam7352]

What & Why

The codebase previously used uuid.UUID (via github.com/google/uuid) for primary keys on users and todos tables, and passed UUIDs through every layer — models, services, handlers, middleware, and the frontend.

The database schema was changed to use SERIAL / auto-increment int primary keys (handled in the migration PR). This PR updates all application code to match that schema change.

Using database-generated integer IDs also simplifies inserts — no need to generate a UUID client-side before writing to the DB.

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How to check failure

On dev (before this PR), with the new int-based schema applied:

1. POST /signup → creates a user, DB returns an integer id
2. POST /login → JWT is minted with user_id as a UUID string (now stale/wrong)
3. GET /todos → middleware tries to parse user_id claim as uuid.UUID, fails → 401 invalid token claims
4. POST /todos → same failure
5. Frontend guest todos use crypto.randomUUID() for local IDs — these are strings, but the API now returns numeric IDs, causing type mismatches in the todo list

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Changes

Backend — models

• User.ID, Todo.ID, Todo.UserID: uuid.UUID → int64
• Removed github.com/google/uuid import from models

Backend — services

• auth_service: Signup uses INSERT ... RETURNING id instead of generating a UUID; JWT user_id claim is now an int64
• todo_service: all method signatures updated from uuid.UUID to int64; AddTodo uses INSERT ... RETURNING id
• TodoService interface updated accordingly

Backend — handlers

• auth.go: extracted signupRequest / loginRequest structs; all status codes use http.Status* constants
• todo.go: uuid.Parse replaced with strconv.ParseInt; request structs extracted; http.Status* constants used throughout

Backend — middleware

• auth.go: JWT user_id claim parsed as float64 (JSON number default) then cast to int64; sets userID as int64 in context; removed uuid dependency

Backend — routes

• Wiring updated to match new handler signatures

Frontend

• types/index.ts: Todo.id widened to number | string to handle both API (numeric) and guest (string) todos
• lib/api.ts: normalise() uses Number() instead of String() for id
• hooks/useTodos.ts: guest todo ID generation replaced with a decrementing integer counter (negative to avoid collisions with server IDs)

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How to verify correction

Backend

# From repo root
cd backend
go build ./...   # must compile with no errors
go vet ./...     # must pass
go test ./...    # unit tests must pass

[TanishqSingla]

You can name it as reqBody

[TanishqSingla]

You mentioned, SERIAL in the design doc but in the code it's mentioned BIGSERIAL.

Why this deviation

[TanishqSingla]

You can't use Number for postgres's Bigserial, they are incompatible

[TanishqSingla]

Why are we changing this here?