Feat/571 mistyped key

[sneumannb5]

What

Closes #571: a mistyped key_id/agent_id no longer yields a 201 Created dead-but-configured-looking binding. Unknown agent → 400; unknown key → warning; list flags already-dead rows.

Why

Neither id is a foreign key (keys in vault, agents in-process registry, not Postgres), so a typo reached zero tools forever while looking identical to a working binding. Existence is now checked in the app layer against the same sources a real request uses — reusing createVerifyOnlyApiKeyStore, so "known == what the endpoint would accept" by construction. Fail-honest: unreadable source = "cannot tell", never a false reject.

Test plan

• middleware typecheck (0 errors) + test (binding suite 58/58, +7 A mistyped key id or agent id yields a fully-configured-looking dead binding #571 cases) + eslint clean
• web-ui vitest 8/8 (3 new + 5 regression) + i18n:check OK + eslint 0 errors

Risk / blast radius

• No schema change; enforcement is app-layer only.
• Response gains an optional warnings[] — clean bindings serialize as before.
• Router existence option is optional — absent ⇒ pre-A mistyped key id or agent id yields a fully-configured-looking dead binding #571 behaviour.
• Stacked on MCP 2026-07-28 readiness (waves 0-6) + #571 binding fix + two-pass security review #550 (feat/mcp-2026-07-28-wave0-wave1), not main.

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