Model conversations by kind and identity

[singular0]

The nullable channel column groups every direct message from every peer into one conversation and
retains only a six-byte peer prefix as sender text. That is enough to avoid losing messages, which
is what it was for, and not enough to build a DM view on.

Reference: src/model/history.cpp:16 and src/model/history.cpp:190.

Tasks:

• Represent a conversation explicitly by kind (channel or direct) and identity.
• Store the peer's public key rather than a prefix of it.
• Migrate the existing rows, including the direct messages already collected under a NULL
  channel.

Blocks the direct-message view; depends on schema versioning being in place first.

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Blocks #1; depends on #29.