feat: add relational cross-links to individual resource endpoints

[AdityaAsopa]

Summary

Adds a _links object to individual record endpoints (/api/spacecrafts/:id and /api/spacecraft_missions/:id) that points to related records in other collections.

What changes

GET /api/spacecrafts/:id now includes:

{
  "id": 10,
  "name": "INSAT-3B",
  ...
  "_links": {
    "self": "/api/spacecrafts/10",
    "mission": "/api/spacecraft_missions/5"
  }
}

GET /api/spacecraft_missions/:id now includes:

{
  "id": 5,
  "name": "INSAT-3B",
  ...
  "_links": {
    "self": "/api/spacecraft_missions/5",
    "spacecraft": "/api/spacecrafts/10"
  }
}

Design decisions

• Only verifiable links — cross-links are resolved by exact case-insensitive name matching between the two datasets. All 64 mission records have a confirmed name match in the spacecrafts dataset.
• No link when no match — if a name match isn't found, _links contains only self. No guessing.
• Collection endpoints unchanged — links only appear on individual :id routes to avoid bloating collection responses.
• _links convention — follows HAL-style hypermedia linking, a widely understood REST pattern.

Notes

This PR builds on feat/individual-id-endpoints (PR #67). The cross-link is only meaningful because both datasets now share consistent names from the normalization in PR #65.

Test plan

• /api/spacecrafts/1 includes _links.self and _links.mission (Aryabhata has a mission entry)
• /api/spacecraft_missions/1 includes _links.self and _links.spacecraft
• A spacecraft with no mission entry has _links with only self
• Collection endpoints (/api/spacecrafts, /api/spacecraft_missions) are unchanged