Add AI-powered evidence description generation

Introduce a background worker that automatically generates
compliance-focused descriptions for uploaded evidence files
using configurable LLM providers. Descriptions are surfaced
across all interfaces: GraphQL API, MCP API, CLI, and the
console UI.

Key changes:
- Multi-provider LLM config with per-agent settings (pointer
  types for Temperature/MaxTokens to preserve zero values)
- Evidence description worker with bounded concurrency
- EvidenceDescriptionStatus typed enum with PostgreSQL enum type
- New `prb evidence` CLI commands (list, view, delete)
- Evidence description displayed in console table and preview
- Migration only marks evidences without files as completed

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/llm/part.go

@@ -26,7 +26,14 @@ type (
 	ImagePart struct {
 		URL string
 	}
+
+	FilePart struct {
+		Data     string // base64-encoded content
+		MimeType string // e.g. "application/pdf", "text/csv", "image/png"
+		Filename string
+	}
 )
 
 func (TextPart) part()  {}
 func (ImagePart) part() {}
+func (FilePart) part()  {}