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/cmd/root/root.go

@@ -25,6 +25,7 @@ import (
 	cmdconfig "go.probo.inc/probo/pkg/cmd/config"
 	cmdcontext "go.probo.inc/probo/pkg/cmd/context"
 	"go.probo.inc/probo/pkg/cmd/control"
+	"go.probo.inc/probo/pkg/cmd/evidence"
 	"go.probo.inc/probo/pkg/cmd/finding"
 	"go.probo.inc/probo/pkg/cmd/framework"
 	"go.probo.inc/probo/pkg/cmd/org"
@@ -73,6 +74,7 @@ func NewCmdRoot(f *cmdutil.Factory) *cobra.Command {
 	cmd.AddCommand(cmdconfig.NewCmdConfig(f))
 	cmd.AddCommand(cmdcontext.NewCmdContext(f))
 	cmd.AddCommand(control.NewCmdControl(f))
+	cmd.AddCommand(evidence.NewCmdEvidence(f))
 	cmd.AddCommand(finding.NewCmdFinding(f))
 	cmd.AddCommand(framework.NewCmdFramework(f))
 	cmd.AddCommand(org.NewCmdOrg(f))