Replace per-field assignment in `update_by_squad_number_async` with a `setattr` loop

[nanotaboada]

Problem

services/player_service.py::update_by_squad_number_async copies the incoming
Pydantic model onto the ORM object one attribute at a time:

player.first_name = player_model.first_name
player.middle_name = player_model.middle_name
player.last_name = player_model.last_name
# ...ten assignments in total

Every field addition needs another line here, and a missed line fails silently
(the field just never updates). This is manual property mapping, a .NET/Java
habit. Note that create_async in the same module already uses the idiomatic
form — Player(**player_model.model_dump()) — so the two paths are
inconsistent.

Proposed Solution

Dump the validated model to a dict and assign with setattr in a loop.
model_dump() uses the Python field names (snake_case), which match the ORM
attribute names, so the mapping is 1:1.

for field, value in player_model.model_dump().items():
    setattr(player, field, value)

The route layer already guarantees player_model.squad_number == squad_number
(400 on mismatch), so including the natural key in the loop is safe.

Suggested Approach

1. Replace the ten assignments with the loop above.
2. Decide whether to keep an explicit player.squad_number = squad_number
   after the loop (documents that the path parameter is authoritative) or rely
   on the route guard — leave a one-line comment either way.
3. Run uv run pytest, including the PUT tests (existing update, mismatch,
   unknown squad number).

Acceptance Criteria

• Field copying is a single loop, not per-field assignments
• Full-replace PUT semantics preserved
• PUT tests pass, including the squad-number mismatch case
• Consistent with create_async's use of model_dump()
• CHANGELOG.md updated

References

• setattr — Python docs
• Pydantic — model_dump
• SQLAlchemy ORM — updating objects