Migrate embedded storage to SQLite

[TheFellow]

Summary

• replace bstore/bbolt persistence with CGO-free embedded SQLite via modernc.org/sqlite
• add WAL-backed same-host multi-process access, immediate writes, busy handling, and serialized versioned migrations
• migrate middleware transactions, SQL filter pushdowns, every domain DAO, tests, and documentation
• add concurrent Store visibility/startup coverage and preserve exact residual filter semantics

Validation

• go test ./...
• go test -race -shuffle=on -count=1 -timeout=5m ./...
• go vet ./...
• go tool arch-lint -config=.arch-lint.yaml
• GOTOOLCHAIN=go1.26.5 go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2 run
• go mod verify
• git diff --check

Migration note

Existing bstore/bbolt database files are not SQLite files. Existing local data must be reseeded or exported with the previous version and imported after upgrading.