MCP `account_verify`: an admin marks a person's email verified, with the same rules as the API #4823

Intent: an admin who knows a person unblocks their verification from the harness.

Done:

• [ ] src/http/mcp.ts has an account_verify tool, placed right after mint_invite's neighbour model_ceiling, taking username. It calls what POST /api/v1/admin/accounts/:username/verify calls (person admins only)
• [ ] docs/MCP.md has its row beside model_ceiling
• [ ] test/mcp-account-verify.test.ts: an admin verifies an unverified person, who can then do what verification gates; a non-admin is refused; an unknown username is not found

Risks: test/mcp-size.test.ts holds tools/list under a 37,000-byte headroom, so keep the description to one short sentence. Other MCP tools are being added in parallel, so put this one exactly where named. Do not change the API routes.