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/admin/api_test.go

@@ -15,14 +15,13 @@ import (
 	"github.com/owncord/server/db"
 	"github.com/owncord/server/permissions"
 	"github.com/owncord/server/service"
-	"github.com/owncord/server/store"
 )
 
 // newTestModService builds a real ModerationService over the test database so
 // PATCH-user ban paths exercise the production authorization (BAN_MEMBERS +
 // role hierarchy) instead of a stub.
 func newTestModService(database *db.DB) *service.ModerationService {
-	st := store.NewSQLiteStore(database)
+	st := database
 	checker := permissions.NewChecker(st)
 	return service.NewModerationService(st, service.NewPermissionService(st, checker))
 }