Document Paddle support in Amplitude, Mixpanel, and Firebase integrations

[eandreeva-twr]

Paddle is becoming a supported store for three realtime integrations: Amplitude, Mixpanel, and Firebase / Google Analytics. Each gets its own optional Paddle credentials in the dashboard, separate from iOS, Android, and Stripe.

Do not merge until the backend change ships. Until then, the Firebase article's current statement that Paddle isn't supported is still accurate.

Changes

• firebase-and-google-analytics.mdx — removed the "Paddle purchases aren't included" limitation. Renamed ### Stripe purchases to ### Web purchases (Stripe and Paddle), since the "Firebase App Instance ID must be set before the purchase fires" rule applies identically to Paddle. Added Paddle to the credential-pair step, the Firebase Console Web-app-entry step, and the note that web purchases use the iOS event map (verified: the backend swaps only the app ID and secret for Paddle, leaving the event map alone).
• amplitude.mdx — added Paddle to the platform list and the field-table row.
• mixpanel.mdx — replaced the vague "token for all platforms or specific platforms" line with the actual per-platform Production/Sandbox breakdown.
• paddle.mdx — added Amplitude, Firebase / Google Analytics, and Mixpanel to the list of integrations that forward Paddle events, plus a line clarifying that Paddle needs its own credentials.

New dashboard fields

Integration	Fields
Amplitude	Paddle Production / Sandbox API key
Mixpanel	Paddle Production / Sandbox token
Firebase / Google Analytics	Firebase app ID for Paddle, Google Analytics secret for Paddle

Follow-up

• Screenshots need recapturing once this is on staging: the Amplitude credentials screenshot and mixpanel.webp won't show the new Paddle fields.
• Adapty Mail already supports Paddle but is missing from paddle.mdx's forwarding list — pre-existing gap, left out of scope here.

Checks

check-mdx-parse and lint-mdx clean. check-links --diff reports 0 broken links; the renamed anchor had one reference, updated in the same file.