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/migrations/20260326T131816Z.sql

@@ -0,0 +1,10 @@
+CREATE TYPE evidence_description_status AS ENUM ('PENDING', 'PROCESSING', 'COMPLETED');
+
+ALTER TABLE evidences
+    ADD COLUMN description_status evidence_description_status NOT NULL DEFAULT 'PENDING',
+    ADD COLUMN description_processing_started_at TIMESTAMPTZ;
+
+UPDATE evidences SET description_status = 'COMPLETED' WHERE evidence_file_id IS NULL;
+
+ALTER TABLE evidences
+    ALTER COLUMN description_status DROP DEFAULT;