[Backend] — Test: `SeasonsService.create` rejects overlapping active seasons

[Olowodarey]

Background

SeasonsService.create (src/seasons/seasons.service.ts) should prevent
creating two seasons with overlapping [starts_at, ends_at] windows. If both
were active simultaneously, leaderboard points would accrue to two seasons at
once. There are no tests verifying this overlap check.

Goal

Add overlap-rejection tests in src/seasons/seasons.service.spec.ts.

Requirements

1. Seed an active season [100, 200].
2. Attempt to create:
   • [150, 250] (starts inside existing) → error.
   • [50, 150] (ends inside existing) → error.
   • [120, 180] (fully inside existing) → error.
   • [200, 300] (starts exactly at existing end) → success.
3. Finalize the existing season; attempt [150, 250] again → success.

Acceptance criteria

• Overlapping seasons are rejected.
• Non-overlapping creation succeeds.

[gloskull]

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