Three SQLite connection issues are fixed:
1. The refactoring in 26c0f71 accidentally dropped _busy_timeout from
the file-based SQLite connection string. Without it, concurrent
transactions get instant SQLITE_BUSY errors instead of waiting.
2. _txlock=immediate forced ALL transactions (including reads) to
acquire the write lock at BEGIN, serializing all database access.
WAL mode makes this unnecessary: readers use snapshots and never
block writers, so the SHARED-to-RESERVED deadlock cannot occur.
3. In-memory shared cache (file::memory:?cache=shared) uses table-level
locking where _busy_timeout is ineffective (returns SQLITE_LOCKED,
not SQLITE_BUSY) and concurrent connections deadlock. Replace with a
temp file using WAL mode for proper concurrency.
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