🧹 Add doubt resolver organization validation

[google-labs-jules]

🎯 What: Replaced a TODO comment in the doubt resolution logic with an actual database query to validate that the resolver (mentor) belongs to the same organization as the mentee's bootcamp.
💡 Why: This enforces organizational boundaries for resolving doubts and avoids exposing an API flaw that trusted the caller entirely. Using sqlc.arg correctly allowed generating an appropriately named Go struct.
✅ Verification: Regenerated sqlc models, updated service tests and successfully ran unit tests and lints locally under Go 1.25.0 as instructed.
✨ Result: Improved security and logic completeness.

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PR created automatically by Jules for task 10487586464920150896 started by @Gautam7352

[google-labs-jules]

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