refactor(server): remove the store abstraction layer (D3)

Deletes Server/store (SQLiteStore, MemStore, the composed Store
interface) and collapses to a single sqlc-backed db package, executing
the prior audit's P4 "single data layer" direction (finding #6).

SQLiteStore was a pure pass-through to *db.DB, so consumers now depend
on narrow interfaces that *db.DB satisfies directly:

  - service.Store   (service/datastore.go, renamed from store/store.go)
  - ws.EventStore   (ws/eventstore.go)
  - plugin.PluginStore (plugin/pluginstore.go)

The event- and plugin-KV methods that lived in the store's SQLite
implementation move into the db package (db/event_queries.go,
db/plugin_queries.go), keeping their raw-SQL form.

Tests: the MemStore-based unit tests now run against a real in-memory
SQLite db opened per-test with migrations applied, via package-local
seed helpers. Fault-injection tests embed a real *db.DB and override the
single method under test, preserving error-path coverage. Full server
suite and sqlc-verify are green.

Docs: audit finding #6 and A-2026-07-06 marked resolved; decisions D3
updated; architecture server.md / data-model.md diagrams and prose
updated to the api -> service -> db layering.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UA17KPvqGBX3XbXYnMf1rA

Server/api/dm_handler_test.go

@@ -14,7 +14,6 @@ import (
 	"github.com/owncord/server/auth"
 	"github.com/owncord/server/db"
 	"github.com/owncord/server/service"
-	"github.com/owncord/server/store"
 )
 
 // ─── DM test schema ─────────────────────────────────────────────────────────
@@ -174,7 +173,7 @@ func dmCreateToken(t *testing.T, database *db.DB, username string, roleID int) s
 
 func buildDMRouter(database *db.DB, broadcaster api.DMBroadcaster) http.Handler {
 	r := chi.NewRouter()
-	svc := service.New(store.NewSQLiteStore(database), auth.NewRateLimiter())
+	svc := service.New(database, auth.NewRateLimiter())
 	api.MountDMRoutes(r, database, svc, broadcaster)
 	return r
 }