Standardize entity ID format: user IDs use underscore, project IDs use hyphen

Summary

Entity ID formats are inconsistent across the codebase:

| Entity | Current Format | Separator |
|--------|----------------|-----------|
| Project | project-c5e8ac67-cb16-4063-a08b-b1d0401fef72 | hyphen |
| User | user_e189e2ea5cb648d2afa11a6d2f3d50f4 | underscore |

Problem

1. Validation complexity - Cannot use a single regex pattern for all entity IDs
2. Developer confusion - Easy to forget which format uses which separator
3. Inconsistent conventions - Hyphens are more standard for UUID-based IDs
4. Parsing complexity - Different split logic needed for different entity types

Proposed Solution

Standardize all entity IDs to use hyphens:

• user-e189e2ea-5cb6-48d2-afa1-1a6d2f3d50f4

Migration Considerations

• This is a breaking change requiring database migration
• All foreign key references need updating
• API consumers need notification
• Consider versioning or deprecation period

Priority

Low - current format works, this is a cleanup/consistency improvement.

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