Fix metrics charts for PostgreSQL

[ulnanlu]

The charts on the metrics page were not working due to datetime data type mismatch between postgresql and the sqilte datetime formats (the metrics code simply assumed datetime as string when implemented during sqlite time).

• Normalize daily metric date keys so PostgreSQL date objects match chart labels.
• Pin Chart.js to a versioned CDN url.
• Add regression coverage for PostgreSQL-style date values.

[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

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