CE Refactor Part 1: create the Models package

[ginaxu1]

For CE Refactor Plan, see https://docs.google.com/document/d/1dLXTVaaMW095MZTpu34cvhbIGenb0ZsBv6-moqvGRXE/edit?tab=t.0#heading=h.tgfrgqvunxrb. This first PR gets the models package in place and ready for PR 2 (Service Package). Models contains domain types, DTOs, constants

Notes:

• Backward compatibility: Maintained via type aliases in engine.go
• No breaking changes: All existing code continues to work

Testing

go mod tidy && go build . && go test ./... -count=1

Related to #337

[ginaxu1]

@sthanikan2000 - now ConsentRecord has 2 expiredAt fields: PendingExpiresAt *time.Time and GrantExpiresAt *time.Timewith PendingTimeoutDuration config (e.g., default "24h" or "P1D").

1. when consent record created, set PendingExpiresAt = now + PendingTimeoutDuration when creating pending consent. Don't set GrantExpiresAt until approved

2. when status changes to "approved", set GrantExpiresAt = now + GrantDuration. Clear PendingExpiresAt

3. when update CheckConsentExpiry, first check pending records: PendingExpiresAt < now → set status to "expired". then check approved records: GrantExpiresAt < now → set status to "expired".

Database migration: need to add pending_expires_at and grant_expires_at columns (nullable)

other changes

• consent_id now uuid like policy_metadata, using GORM to define all consent record fields
• enum for ConsentErrorCode constants

[sthanikan2000]

The reason for suggesting the pointers is when we are referring to these variables without pointers the variable will be the null value of that data type instead of the null data type
Correct the indentations after doing these changes