Vulnerability database total count does not visibly change despite successful ingestion of new advisories

### Current Behavior

Dependency-Track continuously mirrors and ingests new vulnerability/advisory data successfully, but the total vulnerability count displayed in the UI remains static for long periods.

### Steps to Reproduce

1.Example:

* Total vulnerability count remains:
`723815`

Even after:

* successful NVD mirroring
* successful EPSS sync
* new MAL advisories inserted
* new Debian advisories inserted

## Evidence

Mirror logs show successful ingestion:

```text
[NistApiMirrorTask] Mirroring of 14219 CVEs completed
```

Database verification confirms new/updated records are actively inserted:

```sql
SELECT MAX("UPDATED") FROM "VULNERABILITY";

2026-05-13 04:22:27.057+00
```

Recent entries:

```text
MAL-2026-3633
DEBIAN-CVE-2026-5342
```

### Expected Behavior

One of the following should be clarified/improved:

1. UI count should visibly reflect newly inserted advisories/vulnerabilities
2. Documentation should clarify that:

* many mirror operations update existing records in-place
* total counts may remain stable
* advisories are deduplicated/merged
3. Dashboard should expose:

* newly inserted count
* updated count
* merged/deduplicated count

### Dependency-Track Version

4.14.2

### Dependency-Track Distribution

Container Image

### Database Server

PostgreSQL

### Database Server Version

17

### Browser

Google Chrome

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this defect was already reported