fix: Allow '0' in project IDs for reverse proxy

[mdnsk]

DESCRIBE YOUR PR

Added the 0 symbol to the list of allowed characters for project IDs in the reverse proxy configuration. This restores proxy functionality for DSNs like:

https://[email protected]/10
https://[email protected]/100
https://[email protected]/101
etc.

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• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

LEGAL BOILERPLATE

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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