[Feature][Core] Backfill: when timeAfter moves earlier, collect only the missing period instead of restarting full sync #9246

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Use case

A project is first collected with timeAfter = T1. Later the user moves timeAfter to an earlier date T0 < T1 to get more history. Today this restarts the collection from scratch:

• NewCollectorStateManager (backend/helpers/pluginhelper/api/collector_state_manager.go) sees syncPolicy.TimeAfter.Before(state.TimeAfter) and drops to full-sync mode: since = T0, isIncremental = false.
• Every collector then fetches [T0, now], which includes [T1, now], the period that was already collected.
• The REST and GraphQL collectors (api_collector.go, graphql_collector.go: "flush data if not incremental collection") delete all existing raw rows for the scope before fetching.

For large scopes the repeated fetch is the expensive part. GitHub workflow runs and jobs dominate API calls and run time, and the rate limit is shared by every pipeline using the same App or token. A full re-collection also leaves the scope with fewer rows until it finishes.

Description

Add a third collection mode, "backfill", used when the new timeAfter is earlier than the stored one:

1. Collect only [T0, T1]: since = T0, with an upper bound until = state.TimeAfter.
2. Do not flush the existing raw data.
3. On success, store T0 as the new TimeAfter (the existing code already updates it for full syncs).
4. Apply the same rule in SubtaskStateManager, which duplicates the logic.

Collectors need to honour the upper bound. Today only a few call GetUntil() (in the GitHub plugin, only the workflow runs collector); the rest use GetSince() alone. A backfill would add an until filter to each collector's request or stop condition, and fetch child records (reviews, comments, jobs) only for the newly found parents.

Collectors that cannot bound the range could keep today's behaviour (a full re-collect), so this can ship incrementally, starting with the state managers and one or two collectors.

Related issues

None found.

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