backfill: recover never-bridged messages in forward backfill

[Fizzadar]

Problem

Messenger messages that arrive while the bridge is offline can be permanently lost. A reconnect snapshot advances the Meta sync cursor past skipped threads, so their mid-timeline messages are never bridged. A later forward backfill covering the gap then dropped those messages in because ts <= anchor.Timestamp, so the gap was lost forever.

Change

• Remove the forward timestamp DeleteFunc(ts <= anchor) in wrapBackfillEvents.
• Set AggressiveDeduplication: forward on the returned FetchMessagesResponse so bridgev2 deduplicates by message ID — dropping already-bridged messages while retaining never-bridged ones older than the anchor.
• Keep the backward timestamp trim unchanged.

Regression risk (please scrutinize)

The removed forward timestamp trim was an intentional dedup guard. Dedup now relies on bridgev2's ID-based pass. Meta message IDs are deterministic from the network MessageId and the live and backfill paths store under the same ID, so already-bridged messages are still found and dropped; intra-batch dupes are handled by the existing CompactFunc; in-flight messages are covered by the TxnID pending check. Assessed acceptable (high confidence), but maintainer review of the dedup assumption is requested. New minor cost: one indexed GetFirstPartByID lookup per forward-backfill message.

Cross-repo dependency

Requires mautrix/go#517 (skips the forward timestamp prefix-trim in cutoffMessages when AggressiveDeduplication is set). Without that change this PR has no effect, since bridgev2 would re-trim the retained messages by timestamp. Merge order: mautrix/go first, then bump the dependency here.

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