perf(pluto): batch-fetch keys in getAllPrismDIDs to eliminate N+1 queries

[A-Chronicle]

Description

getAllPrismDIDs() previously queried DIDKeyLinks and Keys individually for each prism DID (N+1 pattern — 1 query for DIDs + N queries for links + N queries for keys).

Applied the same batch-fetch pattern used by getAllPeerDIDs():

1. Fetch all prism DIDs (1 query)
2. Fetch all DIDKeyLinks for those DIDs (1 query)
3. Fetch all Keys for those links (1 query)
4. Assemble in memory — O(1) database round-trips

This also removes the private getPrismDIDS() helper that was only used by getAllPrismDIDs.

Fixes #649

Alternatives Considered

• Keeping the existing pattern: simple but degrades with scale
• Going further and also fixing getAllMediators (same N+1 issue) — left as follow-up since it has different structure

Checklist

• My PR follows the contribution guidelines of this project
• My PR is free of third-party dependencies that dont comply with the Allowlist
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked the PR title to follow the conventional commit specification

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud