Store events and config in SQLite

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.

flake.nix

@@ -26,7 +26,7 @@
           pname = "puppy-tracker-server";
           version = "0.2.0";
           src = ./server;
-          vendorHash = null; # no external dependencies
+          vendorHash = "sha256-fqXpr9fV1jeT7503uAjb4jjNPlXfESFnXr7/Uc83d4o=";
           # Pure-Go build for a tiny static binary.
           env.CGO_ENABLED = "0";
           ldflags = [ "-s" "-w" ];
@@ -76,7 +76,7 @@
             exec ${server}/bin/puppy-tracker-server \
               -addr "''${PUPPY_ADDR:-:8080}" \
               -static ${static}/share/puppy-tracker \
-              -data "$data_dir/events.json"
+              -data "$data_dir/puppy.db"
           '');
           meta.description = "Run puppy-tracker locally (data in $XDG_DATA_HOME/puppy-tracker)";
         };