fix: sync directory visibility for individual members

[bokelley]

Summary

• Individual member directory visibility: When a personal account toggles "Show in directories" on their community profile, is_public now syncs to member_profiles (gated on active subscription). Previously only users.is_public was set, so individual members could never appear in the Partner Directory from the profile edit page.
• Slug collision on first save: New members get an auto-generated slug from their name, but the slug field is hidden on first save. If the slug is taken, the save failed with no way to fix it. Now auto-appends a random suffix on collision.
• Slug length validation: Added max length (80 chars) to slug validation to prevent DB column overflow, especially with suffix appending.
• Toggle description: Updated to clarify the toggle controls both community and member directories.

Addresses support tickets #176, #177, #178, #179 reported by Sami MOKRANI (sami@progmaticmotion.com).

Note: Slack access (#177, #178) is operational — no self-service invite flow exists. Sami needs a Slack workspace invite, then email auto-mapping will link the accounts.

Test plan

• TypeScript compiles cleanly (tsc --noEmit)
• All 1050 unit tests pass
• Pre-commit hooks pass (full test suite)
• Verify individual member with active subscription: toggling "Show in directories" on makes them visible in both People Directory and Member Directory
• Verify individual member without subscription: toggling on only affects People Directory, Member Directory stays hidden
• Verify new member with common name: if slug collides, profile save still succeeds with random suffix
• Verify existing members: slug collision retry only triggers on first save, not subsequent edits

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