loopdb: use pgx v5 for postgres

[hieblmi]

• ## Summary

• move Loop’s own Postgres store, migrations, and test fixture to the explicit pgx/v5 stdlib driver
• switch Postgres error handling to the pgx/v5/pgconn types
• bump lnd/kvdb and tidy module files so looprpc no longer pulls in pgx/v4

[gemini-code-assist]

Summary of Changes

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This pull request modernizes the database layer by migrating to the pgx/v5 driver for PostgreSQL. This change ensures better compatibility and performance, standardizes error handling across the application, and streamlines the dependency tree by removing outdated v4 references.

Highlights

• Migration to pgx/v5: Transitioned Loop's Postgres store, migrations, and test fixtures to the explicit pgx/v5 standard library driver.
• Error Handling Refactor: Updated Postgres error handling to utilize the modern pgx/v5/pgconn types.
• Dependency Cleanup: Bumped lnd/kvdb and tidied module files to ensure looprpc no longer pulls in legacy pgx/v4 dependencies.

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[gemini-code-assist]

Code Review

This pull request migrates the database driver and migration framework from the legacy lib/pq and pgx/v4 to pgx/v5, along with updating Go and other dependencies. A critical issue was found in loopdb/postgres.go where the driver name is defined as "pgx/v5". Since the pgx/v5 stdlib driver registers itself as "pgx", using "pgx/v5" in sql.Open will cause a runtime panic.

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[gemini-code-assist]

The pgx/v5/stdlib driver registers itself under the name "pgx", not "pgx/v5". Using "pgx/v5" as the driver name in sql.Open will result in a runtime panic/error: sql: unknown driver "pgx/v5" (forgotten import?).\n\nPlease change the pgxDriver constant to "pgx" to ensure the driver can be correctly loaded.

Suggested change

	pgxDriver = "pgx/v5"
	pgxDriver = "pgx"