feat: add validateCreatorParam middleware for creator route params

[Ghadaffijr]

• Add reusable validateCreatorParam middleware factory in src/middlewares/
• Validates :creatorId and :id route params against alphanumeric/hyphen/underscore pattern
• Returns consistent 400 error via sendValidationError before handler runs
• Wire middleware into creator.routes.ts (GET/PUT /:creatorId/profile)
• Wire middleware into creators.routes.ts (GET /:id/stats, GET /:id/holders)
• 11 unit tests covering valid params, invalid formats, field name reporting

Closes #35

Summary

Adds a reusable validateCreatorParam(paramName) middleware factory in src/middlewares/creator-param.middleware.ts that validates creator route parameters before any handler logic runs
Validates that :creatorId and :id params are non-empty strings matching the allowed pattern (alphanumeric, hyphens, underscores, 1–128 characters)
Returns a consistent 400 error via sendValidationError with the failing field name included in the error details — matching the existing error response shape used across the codebase
Wired into creator.routes.ts on GET /:creatorId/profile and PUT /:creatorId/profile
Wired into creators.routes.ts on GET /:id/stats and GET /:id/holders
Handlers on all covered routes are now simpler — they receive a guaranteed clean param and do not need to defensive-check it themselves

Closes #35

Testing

[x] pnpm lint
[x] pnpm build
[ ] pnpm exec prisma generate when schema or generated types changed
[x] 11 unit tests added covering: valid handles, UUID-style ids, edge characters, missing param, special characters, path traversal, length overflow, and field name in error response

Checklist

[x] Linked issue or backlog item
[x] No secrets or live credentials added
[x] Docs updated if setup or env changed
[x] Change is scoped to one problem

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