fix(collect): Fix the issue where referer might be null

[God-2077]

Handle the case where the referer header could be null, and uniformly use the value from either the referer or referrer header.

#4038

[vercel]

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[greptile-apps]

Greptile Summary

This PR fixes a null-safety issue where request.headers.get('referer') could return null, which would then be passed as the referrer field in the payload — causing downstream issues since the Zod schema expects a string. The fix also adds a fallback to the referrer header name (both spellings).

• Critical bug introduced: Both changed files use JavaScript shorthand { referer } which creates a property named referer, but the Zod schema in src/app/api/send/route.ts expects referrer. Zod's default behavior strips unknown keys, so the referer value will be silently lost for all pixel and link events. The fix should use { referrer: referer } instead.
• The conditional spread ...(referer && { ... }) is a good approach to avoid sending null values.

Confidence Score: 1/5

• This PR introduces a data loss regression — referer data will be silently dropped for all pixel and link collection events.
• While the intent of fixing the null referer is correct, the implementation renames the payload property from referrer to referer, which doesn't match the Zod schema in the API handler. This will cause all referer data to be silently stripped, effectively breaking referrer tracking for pixel and link events.
• Both src/app/(collect)/p/[slug]/route.ts and src/app/(collect)/q/[slug]/route.ts need the field name corrected from referer to referrer.

Important Files Changed

Filename	Overview
src/app/(collect)/p/[slug]/route.ts	Null handling for referer is correct, but the payload field was renamed from referrer to referer, which breaks the Zod schema contract in send/route.ts — referer data will be silently dropped.
src/app/(collect)/q/[slug]/route.ts	Same field name mismatch as the pixel route — referer instead of referrer breaks the API schema, causing referer data to be silently lost for link events.

Sequence Diagram

sequenceDiagram
    participant Browser
    participant CollectRoute as /p or /q Route
    participant SendAPI as POST /api/send
    participant Zod as Zod Schema
    participant DB as Database

    Browser->>CollectRoute: GET /p/:slug or /q/:slug
    Note over CollectRoute: Gets referer header (may be null)
    CollectRoute->>CollectRoute: Build payload with { referer } ❌
    Note over CollectRoute: Field named "referer" instead of "referrer"
    CollectRoute->>SendAPI: POST with payload
    SendAPI->>Zod: safeParse(body)
    Note over Zod: Schema expects "referrer" field<br/>Unknown key "referer" is stripped
    Zod-->>SendAPI: Parsed data (referrer = undefined)
    SendAPI->>DB: saveEvent (referrerPath = undefined)
    Note over DB: Referer data lost silently

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_{Last reviewed commit: 920b81d}

[greptile-apps]

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