feat: add admin trip listing endpoints (list + get)

[diveshpatil9104]

Summary

Completes the trip management feature from README Milestone 2: "Trips: start, end, list active, list historical". PR #55 (merged) implemented start/end so this adds the remaining two read endpoints so admins can see which drivers are currently on the road and review historical trips.

Endpoints

Method	Path	Description
GET	/api/v1/admin/trips	List trips with optional filters (status, vehicle_id, user_id, limit, offset)
GET	/api/v1/admin/trips/{id}	Get a single trip by ID

Both endpoints are protected with authMiddleware(adminMiddleware(...)).

What changed

New files (4):

• trip_list_store.go — TripLister / TripGetter interfaces, compile-time assertions, ListTrips / GetTrip store methods, tripToResponse helper
• trip_list_handlers.go — Handler functions with query param validation
• trip_list_handlers_test.go - ~19 unit tests (mocks, validation, boundary, nullable end_time)
• trip_list_store_test.go - ~9 integration tests (filters, pagination, empty results, not-found, nullable round-trip)

Modified files (5):

• db/query.sql - Added ListTripsFiltered (single parameterized query with sqlc.narg) and GetTripByID
• db/query.sql.go - Auto-regenerated via make generate
• store_trips.go - Added CreatedAt/UpdatedAt to TripResponse, split ErrTripNotFound from ErrActiveTripNotFound
• trip_handlers.go - Updated EndTrip handler to use ErrActiveTripNotFound
• main.go - Registered 2 new admin routes

Design decisions

Single parameterized query over combinatorial explosion: ListTripsFiltered uses sqlc.narg nullable params (WHERE narg IS NULL OR col = narg) instead of separate queries per filter combination. Scales cleanly when future filters are added.

*string / *int64 for optional filters: nil means "no filter" , avoids zero-value bugs where "" or 0 accidentally matches rows.

Split ErrTripNotFound / ErrActiveTripNotFound: The original sentinel said "active trip not found" (correct for EndTrip) but GetTrip reused it for any missing trip. Now each has the right semantics for logging and error wrapping.