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/evidencedescriber/evidencedescriber.go

@@ -0,0 +1,81 @@
+// Copyright (c) 2025-2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package evidencedescriber
+
+import (
+	"context"
+	_ "embed"
+	"fmt"
+
+	"go.probo.inc/probo/pkg/agent"
+	"go.probo.inc/probo/pkg/llm"
+)
+
+//go:embed prompt.txt
+var systemPrompt string
+
+type (
+	Config struct {
+		Model     string
+		Temp      float64
+		MaxTokens int
+	}
+
+	Describer struct {
+		client *llm.Client
+		config Config
+	}
+)
+
+func New(client *llm.Client, cfg Config) *Describer {
+	return &Describer{
+		client: client,
+		config: cfg,
+	}
+}
+
+func (d *Describer) Describe(ctx context.Context, filename string, mimeType string, fileBase64 string) (*string, error) {
+	ag := agent.New(
+		"evidence_describer",
+		d.client,
+		agent.WithInstructions(systemPrompt),
+		agent.WithModel(d.config.Model),
+		agent.WithTemperature(d.config.Temp),
+		agent.WithMaxTokens(d.config.MaxTokens),
+	)
+
+	result, err := ag.Run(
+		ctx,
+		[]llm.Message{
+			{
+				Role: llm.RoleUser,
+				Parts: []llm.Part{
+					llm.TextPart{Text: fmt.Sprintf("Filename: %s", filename)},
+					llm.FilePart{
+						Data:     fileBase64,
+						MimeType: mimeType,
+						Filename: filename,
+					},
+				},
+			},
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("cannot describe evidence: %w", err)
+	}
+
+	text := result.FinalMessage().Text()
+	return &text, nil
+}