[server] Mask email domain in public collection participants

[neeraj-pilot]

Summary

• Previously, the public collection participants endpoint was exposing the full email domain (e.g., ******@gmail.com)
• Now masks both username and domain: first 2 chars of username + *** + @ + first char of domain + *** + last char
• Example: [email protected] → jo**@g*******m

Changes

• Added GetMaskedEmailForPublic function in pkg/utils/email/email.go
• Updated Participants method in pkg/controller/public/comments.go to use the new masking
• Added tests for the new masking function

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid byte slicing to prevent invalid UTF-8 masking

The new masking uses byte indices (s[:2]) on the username (and similarly domain[0]/domain[len-1]), which will split multi‑byte UTF‑8 runes for non‑ASCII local parts or IDN domains (SMTPUTF8). That produces invalid UTF‑8 strings that encoding/json will replace with �, so public participant responses can contain replacement characters instead of a proper masked value. This is a regression from the old masking, which did not slice the string. Consider rune‑aware slicing (e.g., []rune) or utf8 helpers to keep valid characters while masking.

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