Persist engine mounts across seal/unseal cycles

- Add Registry.UnsealAll() that rediscovers mounted engines from the
  barrier on unseal, using stored metadata at engine/_mounts/ with a
  fallback discovery scan for pre-existing mounts (migration path)
- Registry.Mount() now persists mount metadata to the barrier;
  Registry.Unmount() cleans it up
- Call UnsealAll() from both REST and web unseal handlers
- Change Unmount() signature to accept context.Context
- Default CA key size changed from P-384 to P-521
- Add build-time version stamp via ldflags; display in dashboard status bar
- Make metacrypt target .PHONY so make devserver always rebuilds
- Redirect /pki to /dashboard when no CA engine is mounted

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile

@@ -1,7 +1,7 @@
-.PHONY: build test vet clean docker all devserver
+.PHONY: build test vet clean docker all devserver metacrypt
 
 metacrypt:
-	go build -trimpath -ldflags="-s -w" -o metacrypt ./cmd/metacrypt
+	go build -trimpath -ldflags="-s -w -X main.version=$(shell git describe --tags --always --dirty 2>/dev/null || echo dev)" -o metacrypt ./cmd/metacrypt
 
 build:
 	go build ./...