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/anthropic/provider.go

@@ -16,11 +16,13 @@ package anthropic
 
 import (
 	"context"
+	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"net/http"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/anthropics/anthropic-sdk-go"
@@ -185,6 +187,8 @@ func buildMessages(messages []llm.Message) []anthropic.MessageParam {
 							},
 						),
 					)
+				case llm.FilePart:
+					blocks = append(blocks, buildFilePart(p))
 				}
 			}
 			out = append(out, anthropic.NewUserMessage(blocks...))
@@ -460,3 +464,24 @@ func (s *anthropicStream) mapStreamEvent(event *anthropic.MessageStreamEventUnio
 		return llm.ChatCompletionStreamEvent{}, false
 	}
 }
+
+func buildFilePart(p llm.FilePart) anthropic.ContentBlockParamUnion {
+	switch {
+	case strings.HasPrefix(p.MimeType, "image/"):
+		return anthropic.NewImageBlockBase64(p.MimeType, p.Data)
+	case p.MimeType == "application/pdf":
+		return anthropic.NewDocumentBlock(anthropic.Base64PDFSourceParam{
+			Data: p.Data,
+		})
+	case strings.HasPrefix(p.MimeType, "text/"):
+		decoded, err := base64.StdEncoding.DecodeString(p.Data)
+		if err != nil {
+			return anthropic.NewTextBlock(fmt.Sprintf("[file: %s, type: %s, error decoding content]", p.Filename, p.MimeType))
+		}
+		return anthropic.NewDocumentBlock(anthropic.PlainTextSourceParam{
+			Data: string(decoded),
+		})
+	default:
+		return anthropic.NewTextBlock(fmt.Sprintf("[file: %s, type: %s, unsupported format]", p.Filename, p.MimeType))
+	}
+}