feat: AI review proposal editing, confidence/template fields, and filters

[JoaquinBN]

Summary

• Add PUT endpoint for AI review agent to update existing proposals (POST creates, PUT updates)
• Add proposed_confidence and proposed_template fields to SubmittedContribution model with migration
• Validate template_id with PrimaryKeyRelatedField in all serializers to return 400 instead of 500 on invalid template IDs
• Add proposal:accept/reject/more-info search filter for stewards to filter by proposed action type
• Display confidence as color-coded badges in CRM notes panel and submission cards
• Clear proposed_confidence and proposed_template on steward review to prevent stale filter results

Test plan

• POST to /api/v1/ai-review/{id}/propose/ creates a proposal; second POST returns 409
• PUT to /api/v1/ai-review/{id}/propose/ updates an existing proposal; PUT without existing proposal returns 404
• Sending an invalid template_id returns 400 validation error (not 500)
• proposal:reject in steward search bar filters to reject proposals only
• proposal:accept confidence:high composes correctly
• Confidence badge displays in CRM notes panel with correct color
• After steward reviews a submission, proposed_confidence and proposed_template are cleared