Use a `@dataclass` for the `Player` test fake instead of a hand-written `__init__`

[nanotaboada]

Problem

tests/player_fake.py defines the Player test fake as a plain class with an
eleven-parameter __init__ that assigns each argument to self. This is the
pattern a C#/Java developer reaches for (a POCO/POJO with an explicit
constructor), but in Python it is boilerplate the standard library generates
for you.

Two consequences:

• Adding or renaming a player field means editing the parameter list, the
  assignments, and (potentially) call sites by hand.
• Tests serialise the fake with player.__dict__ (e.g. client.post(PATH, json=player.__dict__)), reaching into a dunder attribute instead of a
  documented API.

Proposed Solution

Convert Player to a @dataclass. The decorator generates __init__,
__repr__, and __eq__ from the annotated fields. Replace player.__dict__
with dataclasses.asdict(player), the supported way to get a plain dict from a
dataclass instance.

from dataclasses import dataclass


@dataclass
class Player:
    """Test fake representing a Player."""

    id: str | None = None
    first_name: str | None = None
    middle_name: str | None = None
    last_name: str | None = None
    date_of_birth: str | None = None
    squad_number: int | None = None
    position: str | None = None
    abbr_position: str | None = None
    team: str | None = None
    league: str | None = None
    starting11: bool | None = None

The factory functions (existing_player(), nonexistent_player(),
unknown_player()) already use keyword arguments, so they stay unchanged. In
tests/test_main.py and tests/conftest.py, swap player.__dict__ for
asdict(player) — both produce the same flat, snake_case dict, so request
payloads do not change.

Suggested Approach

1. Add from dataclasses import dataclass; decorate Player; delete the
   explicit __init__.
2. Annotate every field with T | None and a = None default.
3. Replace player.__dict__ with asdict(player) in tests/test_main.py and
   tests/conftest.py (from dataclasses import asdict).
4. Run uv run pytest — all tests must stay green.

Acceptance Criteria

• Player is a @dataclass with no hand-written __init__
• All fields annotated with T | None syntax
• No .__dict__ access remains in the test suite
• Request payloads sent by tests are equivalent to before
• All existing tests pass
• CHANGELOG.md updated

References

• PEP 557 — Data Classes
• dataclasses — Python docs
• dataclasses.asdict