feat(src): configure Discord to Linear backfills

[phorcys420]

Makes the Discord to Linear backfills configurable.

Config

Under linearBridge:

• backfill.enabled (default true): on the first live event of an unmapped #help thread, mirror the thread's full history instead of just that event, so a thread opened while the bridge was off still lands complete.
• deepBackfill.days (default 90): startup import that walks back through #help history. Mirrors every thread active within that many days. -1 imports everything (pages all archived threads, retries through rate limits); 0 disables the startup import.
• deepBackfill.limit (default -1): caps how many threads the startup import mirrors, most recent first. -1 is unlimited; 0 skips the import entirely.

The old backfillAll flag is replaced by deepBackfill.days = -1.

All three are env-overridable and coerced to their real types after load (configmasher hands back strings).

Implementation notes

• backfill moved from a bare boolean to backfill.enabled to leave room for future backfill options.
• Live catch-up (caughtUp) runs on MessageCreate and the debounced ThreadUpdate. It skips already-mirrored posts (Mirror.isMirrored) and routes through withRateLimitRetry, announcing the Linear issue link back to the thread. Reaction events are intentionally excluded to avoid duplicating the catch-up.
• Startup backfill() pages archived threads in batches of 100, stops paging once a windowed import passes the day cutoff, sorts by latest message, applies the day window, then the limit cap.
• deepBackfill.limit === 0 early-returns before any Discord paging.
• backfillThread gained an announce flag: off for the bulk startup import, on for live catch-up.
• Negative days/limit values are treated as the unlimited sentinel (-1); no strict validation added.

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