feat(session-replay): upload replay envelope from the native crash daemon

[tustanivsky]

This PR adds Session Replay product support to the native backend. When a downstream SDK (e.g. sentry-unreal) has staged a recorded clip on disk, the crash daemon now builds a replay_video envelope from it and performs an upload so the clip becomes a first-class Session Replay in Sentry, linked to the crash instead of only being available as a file attachment.

Key Changes

• New internal sentry__session_replay_flush_pending(): scans <database>/replays/ for embedder-staged clips (a replay-<id>.json metadata sidecar + its .mp4), builds the replay_event + replay_recording + video into a replay_video envelope, and hands it to the transport.
• Scope/trace sync: the replay event reuses the crash event's already-applied scope (tags, contexts, release, environment, user, sdk) and lifts its trace_id, read back from <run>/__sentry-event. This makes the replay carry the same tags as the crash and correlate via the same trace.
• Wired into the crash daemon: after the crash report is captured, the daemon calls flush_pending out-of-process, so the replay is sent in the same session as the crash (no next-launch step).
• No new public API - the embedder communicates entirely through the on-disk replays/ convention.

Known Limitations

• Native daemon backend only - not wired for crashpad / breakpad / inproc.

Related items

• Add integration with Session Replay product for native backend sentry-unreal#1445

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