Add enum validation and case-insensitive matching to recommender

[Akshita-2307]

Summary

• Add VALID_LEVELS and VALID_TIME_AVAILABILITY enum validation
• Check parsed skills are non-empty and level/time are valid enums
• Case-insensitive skill matching via .lower() and SKILL_ALIASES (fixes [Bug]: Fix recommendation input validation and remove unused cache logic #673, [Bug]: API payload validation allows invalid enums and empty skill arrays to bypass checks #552, [Bug]: skill matching is case-sensitive — "python" and "Python" treated as different skills #473)

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[Akshita-2307]

Program: GSSoC

[komalharshita]

Looks good to merge!