refactor: split sqlite.go and server.go by domain (FLAW-20)

Pure moves, no behaviour change: the storage layer now has users,
apikeys, thoughts, tasklists, documents, folders, tags, lists, teams and
system files next to sqlite.go (store, schema, migrations); the API has
handlers_*.go per area plus realtime.go and alarms.go next to server.go
(server, router, middleware). Declarations were moved verbatim with an
AST-based tool; imports pruned with goimports.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Doku/architecture.md

@@ -30,8 +30,8 @@ Das Frontend liegt per `go:embed` im Binary ([cmd/server/main.go](../cmd/server/
 | Pfad | Inhalt |
 |---|---|
 | `cmd/server/` | `main.go`, eingebettetes `dist/` |
-| `internal/api/` | `server.go` (Routen, Handler, Alarm-Ticker, SMTP/Discord/Webhook-Versand) |
-| `internal/storage/` | `sqlite.go` (Schema, Migrationen, gesamte Datenlogik) |
+| `internal/api/` | `server.go` (Server, Router mit allen Routen, Middleware, JSON-Helfer); Handler nach Bereich in `handlers_*.go` (`auth`, `account`, `thoughts`, `documents`, `folders`, `lists`, `tasklists`, `teams`, `admin`, `integrations`), `realtime.go` (WebSocket/SSE), `alarms.go` (Ticker und Versand), `docsync.go`, `mail.go`, `netguard.go`, `password_reset.go` |
+| `internal/storage/` | `sqlite.go` (Store, Schema, Migrationen); Datenlogik nach Bereich: `users.go`, `apikeys.go`, `thoughts.go` (inkl. Alarme), `tasklists.go`, `documents.go`, `folders.go`, `tags.go`, `lists.go`, `teams.go`, `system.go` (Einstellungen, Statistik, Export), dazu `visibility.go`, `docsync.go`, `search.go`, `trash.go`, `backlinks.go`, `recurrence.go` u. a. |
 | `internal/domain/` | Strukturen (`models.go`), ID-Erzeugung, Tag-Extraktion |
 | `internal/ws/` | `hub.go` |
 | `internal/collab/` | Kern der Notiz-Synchronisation ohne Datenbank: Blockbaum, Operationen und Konfliktregeln, Sortierschlüssel, Leases, Long-Poll-Benachrichtigung |