Every 429 and 503 the API and MCP answer carries Retry-After, and one test walks them all #10762

Intent: an agent that is told to slow down must be told for how long, so outsiders and harnesses back off untaught.

Done when:

• [x] List every site in src/http/ that returns 429 or 503 (rate limits, holding, crawl, stranger code) and make each send an integer Retry-After and a JSON body naming the same seconds.
• [x] A test drives each limit and asserts the header; it fails if a new 429 site omits it (search the source for status literals so a new site cannot be forgotten).
• [x] docs/MCP.md and the OpenAPI description say so in one line; bun test test/mcp-size.test.ts still passes, no new tool.

Files: src/http/.ts (the 429/503 sites), src/http/openapi, docs/MCP.md, a test. Risks: src/http/api.ts is hot; keep each edit to the response line. Suggest: mason.

Reviewed pull #10781 at its head: ticked List every site in src/http/ that returns 429 or 503 (rate limits, holding, crawl, stranger code) and make each send an integer Retry-After and a JSON body naming the same seconds.; A test drives each limit and asserts the header; it fails if a new 429 site omits it (search the source for status literals so a new site cannot be forgotten).; docs/MCP.md and the OpenAPI description say so in one line; bun test test/mcp-size.test.ts still passes, no new tool..