Proposal: .NET 10 upgrade and optional PostgreSQL support

[wilsonsergio2500]

📌 Context

Hi 👋 — thank you for maintaining Invoicika.

I created this PR to modernize the project and explore a PostgreSQL-based setup, in case it aligns with your roadmap or user needs.
wilsonsergio2500#1

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🚀 What this PR includes

• Upgrade from .NET 6 → .NET 10
• Migration from SQL Server to PostgreSQL
• Updated EF Core packages and tooling
• New clean EF Core migrations for PostgreSQL
• Improved error handling
• Updated documentation

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🤝 Adoption options (totally flexible)

If merging everything isn’t ideal, some alternatives could be:

• Creating a separate tag or branch for PostgreSQL support
• Cherry-picking specific commits (e.g. .NET 10 upgrade only)
• Using this PR as a reference implementation