Build test URLs with f-strings instead of string concatenation

[nanotaboada]

Problem

tests/test_main.py builds request paths by concatenating fragments:

response = client.get(PATH + "squadnumber" + "/" + str(squad_number))
response = client.put(PATH + "squadnumber/" + str(squad_number), json=...)

The file already uses f-strings elsewhere
(f"/players/squadnumber/{player.squad_number}"), so the style is
inconsistent. The + "/" + str(...) form is noisy and fragile — the first
example only produces a valid URL because PATH happens to end in /.

Proposed Solution

Use f-strings consistently, and lift the repeated literal into a constant next
to the existing PATH:

PATH = "/players/"
SQUAD_PATH = "/players/squadnumber"
# ...
response = client.get(f"{SQUAD_PATH}/{squad_number}")

Suggested Approach

1. Add SQUAD_PATH beside PATH.
2. Replace every PATH + ... + str(...) with an f-string.
3. No behavioural change — the generated URLs must be identical. Run
   uv run pytest.
4. tests/test_main.py is excluded from Black — keep the manual formatting
   style.

Acceptance Criteria

• No +-based URL building remains in tests/test_main.py
• Generated URLs are unchanged
• All tests pass
• CHANGELOG.md updated

References

• PEP 498 — Literal String Interpolation
• Python docs — f-strings