Read flushed conversation content with the key when uploading (0.2.13)

[nullarch]

Summary

• sync::open_read_only now carries the database key provider. Under messages/full, events whose content the daemon's periodic flush had moved into encrypted blobs were uploaded as bare metadata: the blob reader had no key and resolve() returned None silently. Seen on the owner's tenant for the events held back by the 2026-09-09 outage.
• A blob that cannot be read (no key, unreadable file) fails the upload with the cursor kept and a clear message, instead of sending stripped events.
• Under messages only said-kinds open their blobs; the inference recompute never opens one.
• Version 0.2.13; CHANGELOG; PROGRESS.

Test plan

• New messages_profile_reads_the_conversation_out_of_flushed_segments_or_holds_the_upload: flush a conversation into blobs under a key file → remove the key → upload held, nothing on the server, cursor 0 → restore the key → text arrives.
• cargo test -p attemptdb-capture --test sync (9), clippy, fmt
• CI matrix

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