fix(api): add JSON tags to TripDetailResp and PlaceDetailsResp

[Ronnie434]

Problem

TripDetailResp and PlaceDetailsResp structs lack JSON struct tags, causing Go's encoding/json to serialize field names in PascalCase (e.g. OriginalPlanID, LatLongs, PlaceDetails). This is inconsistent with every other response struct in the codebase (PlanningResponse, TravelPlan, TimeSectionPlace, etc.) which all use snake_case JSON tags.

This makes it difficult for frontend clients to parse the /plans/:id and /users/plan/:id endpoints reliably, since all other endpoints return snake_case keys.

Before (no JSON tags):

{
  "OriginalPlanID": "abc-123",
  "LatLongs": [[37.31, -121.95]],
  "PlaceDetails": [{"ID": "...", "Name": "...", "PhotoURL": "..."}],
  "ApiKey": "EXPOSED_KEY"
}

After (with JSON tags):

{
  "original_plan_id": "abc-123",
  "lat_longs": [[37.31, -121.95]],
  "place_details": [{"id": "...", "name": "...", "photo_url": "..."}],
}

Changes

• Added json:"snake_case" tags to all fields of TripDetailResp and PlaceDetailsResp
• Marked ApiKey with json:"-" to prevent it from being exposed in API responses (security fix)

Testing

• Verified the fix compiles cleanly with go build ./...
• No behavioral changes other than JSON field naming — all existing data is preserved