fix(governance): namespace flags by content type to prevent id collisions

[Copstud3]

Description

Namespaces governance flags by content type so giveaways and help requests with the same numeric ID cannot share flag records, counts, or auto-suspension behavior.

• Adds a ContentType enum for giveaways and help requests
• Keys flag records and counts by content type and target ID
• Requires content type in flagging and query APIs
• Resolves auto-suspension against only the intended content type
• Adds regression coverage for same-ID coexistence

Closes #411

Checklist

• I have tested my changes locally
• I have updated documentation as needed
• Prisma generation is not applicable
• Prisma migration is not applicable

Testing

• cargo test - 73 passed, 0 failed
• cargo fmt -- --check