feat(tracking): count unique clicks per recipient

[MrFluffen]

Closes #80

What

Clicks and CTR counted every request to /r/[slug], so one person tapping a DM link three times counted as three clicks. They now count once per person per campaign.

• Worker: links sent in a DM (buttons, inline fallback and plain-text DMs) get ?r=<token>. The token is a 22-character HMAC of the recipient's Instagram-scoped ID (hashRecipientId in lib/tracking/server.ts), keyed with NEXTAUTH_SECRET the same way hashClickIp is. The raw ID never appears in the URL.
• Redirect: /r/[slug] stores a well-formed token in the new nullable LinkClick.recipientHash column. A missing or malformed r is ignored, and the redirect behaves exactly as before.
• Counting: the dashboard, the campaign list and the shared report (total, per link and daily) count unique clickers through countUniqueClicks / countUniqueClicksBy in lib/tracking/analytics.ts. A click counts by its token, otherwise by ipHash, otherwise on its own. Older clicks and public-reply links have no token, so they fall back to the IP hash, and nothing needs backfilling.
• Migration: one ADD COLUMN IF NOT EXISTS, identical to what prisma migrate diff generates.

The public reply is left untagged on purpose. It's visible to everyone, so a token there would credit every visitor to the commenter.

Trade-offs worth a look

• The click counts now load id, automationId, recipientHash, ipHash per click and dedupe in memory, rather than running count(). That follows the existing contactsCount pattern (findMany + distinct) and stays small per row. If you'd rather keep it in the database, a COUNT(DISTINCT COALESCE(...)) raw query would also work, and I can switch to it.
• If a recipient forwards the link, clicks by the new person count under the original recipient. That still counts at most once per campaign, so CTR can't be inflated by it.

Checks

• npm test: 314 passed (new tests for token generation, parsing, the redirect and the counting helpers; existing worker and report tests updated for the tokenized URLs).
• npm run lint: clean.
• npm run typecheck / npm run build: both fail on lib/queue/dm-worker.ts:868 (operationId: ... : null passed where sendPostbackOnce expects a string). That error is already on main since fix: stop repeated DMs after ambiguous Meta delivery errors #68, and CI on main fails on it too, so it's unrelated to this PR. With that line bypassed locally, the build passes.

Notes

Feel free to change anything you see fit, or push changes straight to this branch. I hope the issue explains the problem and the idea well enough to give you the full context, and I'm happy to adjust things myself too.

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