doc(analytics): add README.md for GA4 analytics

docs/analytics/README.md

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+# Analytics
+
+AnkiDroid sends opt-in anonymous usage data to Google Analytics 4 (GA4). The
+transport is [google-analytics-kt][lib] — see its README for the library's
+setup, builder API, and the full list of hit types. This document covers
+only the AnkiDroid-side wiring and the questions reviewers tend to have.
+
+[lib]: https://github.com/criticalAY/google-analytics-kt
+
+---
+
+## What AnkiDroid actually sends
+
+Of the six hit types the library supports, AnkiDroid uses three:
+
+| Used by AnkiDroid | Sent from |
+|---|---|
+| `screen_view` | `AnkiDroidUsageAnalytics.sendAnalyticsScreenView(...)` |
+| `event`       | `AnkiDroidUsageAnalytics.sendAnalyticsEvent(...)` |
+| `exception`   | `AnkiDroidUsageAnalytics.sendAnalyticsException(...)` (truncated to 150 chars) |
+
+The complete list of categories and actions lives in
+[`AnalyticsConstants.kt`](../../AnkiDroid/src/main/java/com/ichi2/anki/analytics/AnalyticsConstants.kt).
+We never include card content, deck names, note fields, sync credentials, or
+file paths.
+
+`client_id` is a UUID generated and persisted once per install in a dedicated
+prefs file (`analyticsPrefs`); it is not tied to any Anki account and survives
+profile switches.
+
+---
+
+## Consent when does a hit actually leave the device?
+
+Default state: **opted out**. A hit leaves the device only if all of the
+following are true:
+
+1. The user has ticked the opt-in checkbox in the analytics dialog or
+   settings. Pref key: `analytics_opt_in_v2`, default `false`.
+2. `AnkiDroidUsageAnalytics.optIn` is true every `send…` function starts
+   with `if (!optIn) return`.
+3. The library's own `config.enabled` is true `GaImpl.send()` re-checks
+   this. We pass `enabled = optIn` and call `reinitialize()` when opt-in
+   flips, so this is belt-and-suspenders.
+4. The process won the sampling roll (see below).
+
+If any check fails the hit is dropped before any network I/O.
+
+---
+
+## Performance
+
+Calls are fire-and-forget on `Dispatchers.IO` `sendAsync` returns
+immediately. There is no on-disk queue.
+
+AnkiDroid-specific knobs:
+
+- **Sampling**: configured via `R.integer.ga_sampleFrequency` (production: 10
+  → 10% of installs are in-sample for their process lifetime). `setDevMode()`
+  forces 100%.
+- **Batching**: left off. Each hit is its own POST. See the library's
+  [Limitations][lib-limits] for what batching would buy us.
+
+[lib-limits]: https://github.com/criticalAY/google-analytics-kt#limitations
+
+---
+
+## Network failures
+
+The library wraps every HTTP call in `try/catch` on any exception (server
+down, no internet, captive portal, timeout) it returns
+`GaResponse(statusCode = -1)`, logs at ERROR, and the hit is gone. No retry,
+no backoff, no offline replay see the library [Limitations][lib-limits].
+Default timeouts are 10s connect / 30s read, blocking only the IO coroutine.
+
+---
+
+## Can it crash AnkiDroid?
+
+**Send path: no.** Both `sendAsync`'s outer launch and the inner OkHttp call
+have `try/catch (Exception)`.
+
+**Init path: the only realistic risk.** `AnkiDroidUsageAnalytics.initialize`
+is called from `AnkiDroidApp.onCreate()` and runs
+`GoogleAnalytics.builder { ... }`, which constructs an `OkHttpClient`. The
+builder does not validate `measurementId`/`apiSecret` today empty strings
+just produce bad URL params, not exceptions. But to stay safe against future
+library changes, we should wrap `initialize` in
+`runCatching { ... }.onFailure { Timber.e(it) }` so a bad config can never
+prevent app start.
+
+---
+
+## Debug logging seeing what would be sent
+
+The library uses [`io.github.oshai:kotlin-logging`][klog] (an slf4j facade)
+and already logs the full request body, response, sampling decision, and
+drop reasons. To see them in AnkiDroid debug builds, add an slf4j → Timber
+bridge in the debug flavor (e.g. the `slf4j-timber` artifact, or a small
+custom binding). No library change required, no-op in release.
+
+[klog]: https://github.com/oshai/kotlin-logging
+
+---
+
+## Setting up your own GA4 property
+
+For getting a measurement ID + API secret from the GA4 admin panel, follow
+the library's [Prerequisites section][lib-prereqs]. Once you have them, plug
+them into AnkiDroid like so:
+
+| Value | Where it goes |
+|---|---|
+| Measurement ID (`G-XXXXXXXX`) | `AnkiDroid/src/main/res/values/analytic_constants.xml` → `ga_trackingId` |
+| API secret | `local.properties` → `ANALYTICS_API_KEY=...` (read at compile time into `BuildConfig.ANALYTICS_API_KEY`, see [`AnkiDroid/build.gradle`](../../AnkiDroid/build.gradle)) |
+
+Builds without an `ANALYTICS_API_KEY` fall back to `DUMMY_API_XXX`, which GA
+rejects at ingest contributor builds can't accidentally write to our
+production property.
+
+[lib-prereqs]: https://github.com/criticalAY/google-analytics-kt#prerequisites
+
+### Keeping dev traffic out of prod
+
+A signed dev build with the real `ANALYTICS_API_KEY` would hit our
+production GA4 property. Two reasonable options when we decide to address
+this:
+
+- A separate GA4 property for the `debug` flavor (cleanest, needs another
+  secret).
+- Set `debug = true` on the library config in debug builds that routes to
+  GA's validation-only endpoint, which doesn't record.
+
+Not implemented yet.