Count backfilled glucose in the daily analytics summary

[ps2]

Stacked on #2574. It needs LoopKit/LoopKit#646 for StoredGlucoseSample.isBackfill and receivedDate.

The daily summary's cgmFetchCount only counted readings fetched by polling. CGMs that push readings, through the cgmManager(_:hasNew:) path, were never counted. It's replaced by counts of CGM readings actually stored, taken from what addGlucoseSamples returns. Duplicates the store drops aren't counted.

• glucoseCount: CGM readings stored that day (user-entered readings excluded)
• backfillCount: how many of those were tagged backfill by the CGM plugin
• maxBackfillAgeMinutes: the longest gap between a backfilled reading's time and when Loop received it

The per-fetch "CGM Fetch" hook is removed.

Plugin PRs that set the tag:

• Tag backfill readings G7SensorKit#94
• Tag backfill readings CGMBLEKit#226
• Tag historical and clinical gap-fill readings as backfill LibreLoop#23
• Tag history readings as backfill LibreTransmitter#57
• Tag catch-up readings as backfill EversenseKit#5

Uploaders:

• Add isBackfill to GlucoseEntry NightscoutKit#4
• Upload the backfill tag on glucose entries NightscoutService#39
• Include the backfill tag in glucose datum payloads TidepoolService#43

Tested by building the LoopWorkspace scheme with all of the above.

[ps2]

This merged into the stacked base branch after #2574 was squash-merged, so it didn't reach next-dev. Re-opened against next-dev as #2578.