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

tests/test_versioning.py

@@ -0,0 +1,67 @@
+"""Tests for plugin versioning: upgrade, downgrade, compatibility checks."""
+
+from __future__ import annotations
+
+import pytest
+
+from app.plugins.semver import SemVer
+from app.plugins.manifest import PluginManifest
+
+
+class TestManifestVersioning:
+    def test_manifest_has_min_app_version(self):
+        """PluginManifest has min_app_version field."""
+        m = PluginManifest(
+            name="test",
+            version="1.0.0",
+            display_name="Test",
+        )
+        assert hasattr(m, "min_app_version")
+        assert m.min_app_version == "0.0.0"
+
+    def test_manifest_with_min_app_version(self):
+        """PluginManifest can set min_app_version."""
+        m = PluginManifest(
+            name="test",
+            version="1.0.0",
+            display_name="Test",
+            min_app_version="1.5.0",
+        )
+        assert m.min_app_version == "1.5.0"
+
+    def test_manifest_extra_forbid_still_works(self):
+        """Manifest still rejects unknown fields."""
+        with pytest.raises(Exception):
+            PluginManifest(
+                name="test",
+                version="1.0.0",
+                display_name="Test",
+                unknown_field="value",
+            )
+
+
+class TestVersionComparison:
+    def test_upgrade_detection(self):
+        """SemVer correctly detects upgrades."""
+        old = SemVer.parse("1.0.0")
+        new = SemVer.parse("1.1.0")
+        assert new.is_upgrade_from(old)
+        assert not old.is_upgrade_from(new)
+
+    def test_downgrade_detection(self):
+        """SemVer correctly detects downgrades."""
+        old = SemVer.parse("1.1.0")
+        new = SemVer.parse("1.0.0")
+        assert new.is_downgrade_from(old)
+
+    def test_breaking_change_detection(self):
+        """Major version change is a breaking change."""
+        assert SemVer.parse("1.0.0").is_breaking_change(SemVer.parse("2.0.0"))
+        assert not SemVer.parse("1.0.0").is_breaking_change(SemVer.parse("1.5.0"))
+
+    def test_compatibility_check(self):
+        """Version compatibility works correctly."""
+        assert SemVer.parse("1.5.0").is_compatible_with(SemVer.parse("1.0.0"))
+        assert not SemVer.parse("1.0.0").is_compatible_with(SemVer.parse("1.5.0"))
+        assert SemVer.parse("2.0.0").is_compatible_with(SemVer.parse("1.0.0"))
+        assert not SemVer.parse("1.0.0").is_compatible_with(SemVer.parse("2.0.0"))