feat: Plugin-System Umbau — 6 Phasen komplett abgeschlossen

Phase 1: Contracts konsequent nutzen
- 12 neue contracts.py erstellt (alle 19 Plugins haben jetzt contracts)
- 4 bestehende contracts.py an zentrale ContractRegistry angepasst
- Alle 19 Plugins haben on_deactivate mit Contract-Unregister
- 0 echte problematische INTER-Plugin Imports

Phase 2: Hooks/Filters-System
- app/core/hooks.py (HookRegistry mit actions + filters)
- 15 Hook-Punkte in Core-Services (contact, auth, mail, calendar, user, dms)
- BasePlugin.on_deactivate meldet alle Hooks ab

Phase 3: Plugin-Isolation
- scripts/check_cross_plugin_imports.py (Linting-Regel)
- .github/workflows/check-cross-plugin-imports.yml (CI/CD)
- .pre-commit-cross-plugin.yaml (Pre-commit Hook)
- 155 Dateien geprueft, 0 Verstoesse

Phase 4: Plugin-Versioning
- app/plugins/semver.py (SemVer mit Parse, Compare, Pre-release)
- migration_runner.py erweitert: run_migration_down, rollback_to_version
- manifest.py: min_app_version Feld
- registry.py: App-Version-Compatibility-Check bei Installation
- GET /api/v1/plugins/updates Endpoint

Phase 5: Marketplace-Vorbereitung
- app/plugins/signature.py (Ed25519 Signatur-Validierung)
- app/plugins/quarantine.py (Plugin-Quarantine mit Validierung)
- app/models/plugin_allowlist.py + Migration 0046
- manifest.py: author, license, homepage, icon, screenshots, changelog, marketplace_tags, price
- registry.py: discover_external(), discover_all()
- POST /api/v1/plugins/install-marketplace (deaktiviert)

Phase 6: Manifest-Anpassung
- manifest.py: 12 neue Felder + SemVer/Hook-Name Validierung
- MANIFEST_SCHEMA_DOC aktualisiert
- Alle 19 Plugin-Manifeste aktualisiert
- Frontend PluginUiManifest Typ erweitert

Zusaetzliche Bug-Fixes:
- test_sample-Modul erstellt
- conftest.py Deadlock-Prevention
- SESSION_COOKIE_SECURE=true
- dump.rdb aus Git entfernt + .gitignore
- backup.py datetime.utcnow -> func.now()
- system_settings.py JSONB-Import nach oben
- tax.py Mapped[float] -> Mapped[Decimal]
- notification.py type_key-Laengen vereinheitlicht

Tests: 91 neue Tests, alle bestanden

app/plugins/builtins/report_generator/contracts.py

@@ -0,0 +1,57 @@
+"""Public contract for the report_generator plugin.
+
+Exposes models and PDF generation functions for other plugins.
+"""
+
+from __future__ import annotations
+
+from app.plugins.builtins.contracts import get_contract_registry
+from app.plugins.builtins.report_generator.models import (
+    ReportInstance,
+    ReportTemplate,
+)
+from app.plugins.builtins.report_generator.pdf_generator import (
+    PRESET_META,
+    PRESET_TEMPLATES,
+    generate_pdf,
+    generate_pdf_from_template_content,
+    generate_preset_report,
+    generate_print_pdf,
+    get_preset_list,
+    render_template_file,
+    render_template_string,
+)
+
+
+class ReportGeneratorContract:
+    """Public API surface for the report_generator plugin."""
+
+    contract_name = "report_generator"
+
+    # ─── models ───
+    ReportTemplate = ReportTemplate
+    ReportInstance = ReportInstance
+
+    # ─── pdf_generator ───
+    generate_pdf = staticmethod(generate_pdf)
+    generate_print_pdf = staticmethod(generate_print_pdf)
+    generate_preset_report = staticmethod(generate_preset_report)
+    generate_pdf_from_template_content = staticmethod(generate_pdf_from_template_content)
+    render_template_file = staticmethod(render_template_file)
+    render_template_string = staticmethod(render_template_string)
+    get_preset_list = staticmethod(get_preset_list)
+    PRESET_META = PRESET_META
+    PRESET_TEMPLATES = PRESET_TEMPLATES
+
+
+# ─── self-registration ───
+
+_contract = ReportGeneratorContract()
+get_contract_registry().register("report_generator", _contract)
+
+
+__all__ = [
+    "ReportGeneratorContract",
+    "ReportTemplate",
+    "ReportInstance",
+]