Preserve paginated thread metadata across resumes

[copyberry]

Preserve paginated thread metadata across resumes

Why

Paginated rollout history may contain only a bounded suffix. Deriving display
metadata from that suffix can replace the thread's original preview, title, and
first user message when the thread is read or resumed.

What changed

• Use SQLite as the canonical source of display metadata for paginated thread
  reads, including reads by an explicit rollout path.
• Seed resume metadata tracking from the persisted values so appended history
  fills only missing fields instead of overwriting existing ones.
• Prefer an explicitly requested rollout path over a stale path stored in
  SQLite while retaining the canonical metadata.

Testing

Added coverage for reads by ID and path, bounded-history resumes across app
server restarts, preservation of existing metadata, initialization of missing
fields, and stale persisted rollout paths.