fix(api): include profile_id in profile creation response

[sehr-abrar]

Summary

Profile creation returned the identifier only as id, but clients expect a
profile_id field to use in follow-up requests like GET /profiles/{profile_id}.
This PR adds profile_id to the ProfileResponse schema, mapped from the existing
id. The id field is unchanged, so the change is additive and backward-compatible.

Issue

Closes #78

Changes

• api/schemas/profile.py: added a profile_id computed field to ProfileResponse
  that returns the model's id. A plain profile_id: UUID field would not work
  because the schema uses from_attributes and the ORM object has .id, not
  .profile_id, so the value is derived from id instead.
• tests/unit/test_profile_schema.py: new test module (5 tests) for the new field
  and backward compatibility.

Testing

• Unit tests pass (make test-unit) (see pre-existing failures note)
• Integration tests pass (make test-integration) (N/A; the repo has no integration tests yet, so this collects 0 tests)
• Linter passes (make lint) (see pre-existing failures note)
• Type checker passes (make typecheck)
• New/updated tests cover the changes

New tests in tests/unit/test_profile_schema.py cover: profile_id is present in
the response, it equals both the source id and the response id, id is still
present for backward compatibility, profile_id serializes to the same UUID string
as id in JSON, and profile_id is present when all optional fields are None.

Manual verification:

1. git checkout fix/78-profile-response-missing-profile-id
2. python -m pytest tests/unit/test_profile_schema.py -v (expect 5 passed)
3. With the app running, POST /profiles and confirm the JSON body now includes
   profile_id alongside id.

Pre-existing failures (unrelated to this PR): main already fails checks in modules
this PR does not touch. On main, make test-unit reports 55 failing tests (e.g.
test_tech_detector.py, test_skill_extractor.py) and make lint reports about 161
ruff errors in those same modules. After this PR the counts do not increase (unit
failures 55 to 53, with the 5 new tests passing), and the two files I changed pass
ruff, black, and mypy individually. This PR does not try to fix the pre-existing failures.

Notes for Reviewers

• Kept id on purpose: frontend/src/components/ProfileForm.tsx reads profile.id
  from the create response, so removing or renaming it would break the frontend.
  The change only adds profile_id.
• ProfileResponse is shared by the create, get, and update endpoints in
  api/routes/profiles.py, so profile_id now appears in all three responses.
  This is intended and safe since it only adds a field.