feat: vulnerability scanning pipeline, matcher, scheduler and API

Completes tasks 10-15 and fixes what was outstanding:

- vulndb.Pull implemented with oras-go, streaming the ~50MB layer and
  staging both files before replacing either, so a failed pull leaves the
  previous database intact rather than a half-written one.
- db.go: Vulnerability.Severity is a string, not trivy Severity, so the
  int conversion did not compile. Severity now resolves vendor (highest
  when vendors disagree) then NVD then unknown, and CVSS is read too.
- findings.go: added sweepFixedFindings plus the fleet query, severity
  counts, rescan flag and accept/unaccept the API needs.
- vulnrules.go: added rule CRUD and the digest builder. ResolveTargets
  returns []models.Server, not []string, so filterByServers was wrong.
- api/vulnerabilities.go was an empty file while handlers.go registered
  twelve routes against it; written, grouped by CVE.
- shared/mail: added the missing sender. The templates were orphaned and
  the HTML one was a copy of the text one, defining "subject" (which
  html/template would escape) and emitting no markup. render.go parses
  every template in init(), so a bad one panics server, admin and sitesvc
  at boot — go build never runs init(), which is why nothing complained.
- notify: digests dispatch through their own path so SMTP gets the digest
  template rather than arriving dressed as a monitor alert.

server/go.mod

@@ -15,11 +15,15 @@ require (
 )
 
 require (
+	github.com/aquasecurity/trivy-db v0.0.0-20260713131703-4be526083c54 // indirect
 	github.com/hyperboloide/lk v0.0.0-20251220053519-b291812e3216 // indirect
 	github.com/knqyf263/go-apk-version v0.0.0-20200609155635-041fdbb8563f // indirect
 	github.com/knqyf263/go-deb-version v0.0.0-20241115132648-6f4aee6ccd23 // indirect
 	github.com/knqyf263/go-rpm-version v0.0.0-20240918084003-2afd7dc6a38f // indirect
+	github.com/opencontainers/go-digest v1.0.0 // indirect
+	github.com/opencontainers/image-spec v1.1.1 // indirect
 	github.com/robfig/cron/v3 v3.0.1 // indirect
+	oras.land/oras-go/v2 v2.6.2 // indirect
 )
 
 require (