chore: move PostgreSQL Grafana dashboard into dashboards/grafana/

[seanthegeek]

Summary

The PostgreSQL Grafana dashboard (added in #667) shipped at the repo-root grafana/ directory by oversight. Every other dashboard source lives under dashboards/ (opensearch/, grafana/, splunk/), so this moves it next to the existing Grafana dashboard:

• grafana/Grafana-DMARC_Reports-PostgreSQL.json → dashboards/grafana/Grafana-DMARC_Reports-PostgreSQL.json (pure rename, file unchanged)
• Lists it in dashboards/README.md alongside the other shipped dashboards
• Fixes the CHANGELOG.md path reference
• Removes the now-empty top-level grafana/ directory

The only file references to the old path were the CHANGELOG line (fixed); the grafana/grafana hit in docker-compose.dashboard-dev.yml is the Docker image name, not a path. No packaging impact — the wheel ships only parsedmarc/.

Test plan

• git records a 100% rename
• no remaining references to the old grafana/Grafana-DMARC_Reports-PostgreSQL.json path

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.54%. Comparing base (caac8e6) to head (d335834).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #780   +/-   ##
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  Coverage   85.54%   85.54%           
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  Files          17       17           
  Lines        4628     4628           
=======================================
  Hits         3959     3959           
  Misses        669      669           

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