[#494] Generate OpenAPI 3.0 spec from route annotations and serve at `GET /api/docs`

[AdaBebe0]

Backlog reference: #494 · docs

Summary

Generate OpenAPI 3.0 spec from route annotations and serve at GET /api/docs

Context

Reference material a new contributor or self-hosting operator needs that isn't already covered by a category-specific doc: the OpenAPI spec, the deployment guide, and the onboarding walkthrough.

The path to a public, mainnet launch: KYC/compliance groundwork, price/localization/mobile polish, performance hardening, documentation, and the final production go-live checklist.

Acceptance Criteria

• Generate OpenAPI 3.0 spec from route annotations and serve at GET /api/docs — implemented as described above.
• The documentation is verified accurate against the current code, not just written to match intent.
• The new doc is linked from README.md or the relevant docs index so it is discoverable.

Definition of Done

• Implemented on a branch and opened as a PR referencing this issue (Closes #494 won't auto-link since this is a repo-internal reference — mention the issue number GitHub assigned instead).
• CI is green (lint, typecheck, tests, build).
• Reviewed against CONTRIBUTING.md conventions (Conventional Commits, code style).
• CHANGELOG.md updated under [Unreleased] if the change is user- or API-visible.
• Merged to main.

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Generated from the CoopLumen backlog (issue.md). See CONTRIBUTING.md for the contribution workflow.