Record a conversion for a click

EAS-83: a Server-Sent Events stream of new clicks on this link as they're recorded. Layered on top of the existing ~500ms click-batch pipeline (never a separate capture path): the connection emits `data: <Click>` for each click as its batch is durably persisted (bot clicks excluded, same as recentClicks), one JSON object per click, plus `: ping` heartbeat comments roughly every 25s. One-way and stateless — any replica can serve any client. Same readable scope as GET /v1/links/{id}: the owner, or anyone in the organization when the link's visibility is "org". EAS-83 follow-up: each Click also carries `clicksUsed`/`qrScansUsed` — the same live Redis usage-cap counts GET /v1/links/{id} returns at the top level — so a client can track the Click/QR-scan-limit stats live instead of only refreshing them on the next full reload. Both are omitted if the usage limiter isn't wired server-side (same best-effort degrade as GET's own clicksUsed/qrScansUsed). Open streams are capped per link and per user on each replica; past a cap the response is 429 with a plain-language message and the client retries on its own. The server ends the stream when the caller's access token expires, so a client reconnects (refreshing its token) and refetches — clicks in the gap are not replayed. Every couple of minutes the server also re-checks that the caller still has access (their identity has not changed, and the link still exists and is readable by them) and ends the stream if not.