Create db if not exists

[fbac]

Closes #466

[graphite-app]

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[macroscopeapp]

Create the database only if it does not exist in pkg/db/pgx.go:createNamespace by adding an explicit existence check

• In pkg/db/pgx.go, replace error-code based duplicate handling with an explicit SELECT EXISTS(...) check against pg_database.
• Return a distinct error when the existence check query fails and return early when the database already exists.
• On creation, call CREATE DATABASE only after a false existence check and return a formatted error on failure.
• Remove unused errors and PostgreSQL error-code handling imports from pkg/db/pgx.go.

📍Where to Start

Start with the createNamespace function in pkg/db/pgx.go.

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Macroscope summarized e0d3bef.