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/coredata/evidence_description_status.go

@@ -0,0 +1,68 @@
+// 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 coredata
+
+import (
+	"database/sql/driver"
+	"fmt"
+)
+
+type (
+	EvidenceDescriptionStatus string
+)
+
+const (
+	EvidenceDescriptionStatusPending    EvidenceDescriptionStatus = "PENDING"
+	EvidenceDescriptionStatusProcessing EvidenceDescriptionStatus = "PROCESSING"
+	EvidenceDescriptionStatusCompleted  EvidenceDescriptionStatus = "COMPLETED"
+)
+
+func (s EvidenceDescriptionStatus) MarshalText() ([]byte, error) {
+	return []byte(s.String()), nil
+}
+
+func (s *EvidenceDescriptionStatus) UnmarshalText(data []byte) error {
+	val := string(data)
+
+	switch val {
+	case EvidenceDescriptionStatusPending.String():
+		*s = EvidenceDescriptionStatusPending
+	case EvidenceDescriptionStatusProcessing.String():
+		*s = EvidenceDescriptionStatusProcessing
+	case EvidenceDescriptionStatusCompleted.String():
+		*s = EvidenceDescriptionStatusCompleted
+	default:
+		return fmt.Errorf("invalid EvidenceDescriptionStatus value: %q", val)
+	}
+
+	return nil
+}
+
+func (s EvidenceDescriptionStatus) String() string {
+	return string(s)
+}
+
+func (s *EvidenceDescriptionStatus) Scan(value any) error {
+	val, ok := value.(string)
+	if !ok {
+		return fmt.Errorf("invalid scan source for EvidenceDescriptionStatus, expected string got %T", value)
+	}
+
+	return s.UnmarshalText([]byte(val))
+}
+
+func (s EvidenceDescriptionStatus) Value() (driver.Value, error) {
+	return s.String(), nil
+}