Add docs for server environment variable reference with all required and optional vars

[Chucks1093]

Summary

The server reads configuration from environment variables but there is no single reference listing all of them, their types, whether they are required, and what the defaults are for optional ones. Contributors and operators have to read the source to find this information.

Scope

• Document all environment variables grouped by category: database, Stellar RPC, rate limiting, webhook delivery, indexer, logging
• For each variable: name, type, required/optional, default value if optional, and a one-line description
• Add a note on which variables must be rotated regularly for security (RPC keys, signing secrets)

Acceptance Criteria

• All variables documented with type, required flag, and default
• Variables grouped by category
• Security rotation note included for sensitive vars

ETA: 12 hours

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Coordinate on Telegram

[SamixYasuke]

@SamixYasuke has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, I'd like to work on this by documenting all environment variables grouped by category (database, Stellar RPC, rate limiting, webhooks, indexer, logging) with type, required/optional, defaults, and a security rotation note for sensitive vars. Can I get this assigned? Thanks

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[Oluwasuyi-Timilehin]

@Oluwasuyi-Timilehin has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, please assign this to me. I would get started immediately

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[Lazaruster]

@Lazaruster has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i will love to work on this issue

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[parkerwinner]

@parkerwinner has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, am familiar with server issues and can resolve this issue robustly, kindly assign it to me

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This issue has been completed by @parkerwinner as part of the Stellar Wave Program's 6th Wave 🥳

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