feat(config): stricter validation for log_level and printer ports

[strausmann]

Background

Deferred from PR #45 (Task 1.4) review. Multiple reviewers flagged the
absence of runtime validation on log_level and printer ports.

Acceptance criteria

• log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"]
• ql820_port, pt750w_port validated to 1 <= port <= 65535 via Field(ge=1, le=65535)
• Optional: validate webhook_api_key rejects whitespace-only strings of 32+ chars (separate minor finding from same review)
• Unit tests cover each new validation

Plan reference

docs/superpowers/plans/2026-05-11-label-printer-hub.md Phase 1 — not in the
original plan but a clean extension of Task 1.4.