Make DB pool sizing configurable

[hman38705]

Area: Backend
Files: services/api/src/db.rs, services/api/src/config.rs

Problem: DB pool min/max are hard-coded.

Acceptance Criteria:

• Pool sizes and timeouts are env-configurable.
• Safe defaults remain in place.
• Config docs updated.

[Danto1606]

@Danto1606 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

i would like to work on this

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[drips-wave]

Congratulations, @Danto1606! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @Danto1606: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @Danto1606 as part of the Stellar Wave Program's 3rd Wave 🥳

Moderator's reason: [Auto-assessed] The contributor successfully addressed the issue by making the DB pool configuration environment-driven. The PR includes code changes for parsing variables, documentation updates, and safe defaults, and it has already been merged by a maintainer.

😎 @Danto1606: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

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ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.