Replace the JSON-file event and config stores with a SQLite database
(modernc.org/sqlite, pure-Go so the static build keeps CGO_ENABLED=0).
Last-write-wins now rides on the upsert's WHERE clause rather than a
Go-side map compare; the sync protocol and HTTP handlers are unchanged.
On first start the server auto-imports any legacy events.json/config.json
sitting in the data dir, renaming them to *.imported. The -data flag now
points at puppy.db; photos still live on the filesystem alongside it.
Alexander Heldt