Fix/streak freeze unlimited stockpile

[Midoriya-w]

Closes #1476

Summary

Added a pending freeze count check before creating a new streak freeze.

Changes

• Count existing pending freezes for the authenticated user
• Enforce a maximum of 1 pending freeze
• Return HTTP 409 when the limit is exceeded
• Prevent users from stockpiling unused freezes

Result

Users can no longer accumulate unlimited pending freezes and are restricted to a single pending freeze at a time.

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[Priyanshu-byte-coder]

Closing as duplicate: #1542 addresses the same streak freeze stockpile issue and additionally fixes the response status code (200 vs 201 for already-existing freezes). #1542 is the more complete fix.