Allow audiusAppUrl to be overridden via env var

[rickyrombo]

Summary

Cfg.AudiusAppUrl was hardcoded inside the per-env switch in config.init() for dev, stage and prod, so it could not be pointed at a non-default frontend without a code change. Developer apps running against a local override or preview deploy got the wrong redirect_uri base.

This applies an audiusAppUrl env override after the switch, alongside the existing archiverNodes and antiAbuseOracles overrides that solve the same problem for values the switch also sets per-env:

// Override the Audius app base URL when set, so developer apps running against a
// non-default frontend (local override, preview deploy) get the right redirect_uri base.
if v := os.Getenv("audiusAppUrl"); v != "" {
    Cfg.AudiusAppUrl = strings.TrimSuffix(v, "/")
}

Placing it after the switch means it covers every env and cannot be silently missed when a new env case is added — the earlier revision of this PR guarded dev and prod individually, and a stage default added to main in the meantime went unguarded.

The trailing slash is trimmed because every consumer builds URLs as base + "/..." (v1_oauth.go, v1_sitemaps.go, v1_users_sales_download.go, v1_users_purchases_download.go), so a trailing slash would produce //.

Rebased onto current main — the previous revision was 246 commits behind and conflicting.

Test plan

go build, go vet and gofmt are clean. A standard unit test can't cover this: init() runs at package load, before t.Setenv could take effect. Verified instead by running the real package across the env matrix:

ENV	audiusAppUrl	Cfg.AudiusAppUrl
dev	(unset)	http://localhost:3000
dev	http://custom.example	http://custom.example
stage	(unset)	https://staging.audius.co
stage	http://custom.example	http://custom.example
prod	(unset)	https://audius.co
prod	https://preview.audius.co	https://preview.audius.co
prod	https://trailing.example/	https://trailing.example

Note

Cfg.AudiusdURL has the same latent bug — read from os.Getenv("audiusdUrl") in the Cfg initializer, then unconditionally overwritten in all three switch branches. Left alone as out of scope.

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