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/automation/contracts.py

@@ -0,0 +1,82 @@
+"""Public contract for the automation plugin.
+
+Exposes models, services, scheduler, and agent communication for other plugins.
+"""
+
+from __future__ import annotations
+
+from app.plugins.builtins.contracts import get_contract_registry
+from app.plugins.builtins.automation.models import (
+    AgentDefinition,
+    AgentRun,
+    AgentVersion,
+    AutomationCronJob,
+    AutomationDefinition,
+    AutomationRun,
+    AutomationVersion,
+)
+from app.plugins.builtins.automation.services import (
+    AgentService,
+    AutomationService,
+    CronJobService,
+    RunLogService,
+)
+from app.plugins.builtins.automation.agent_runner import run_agent
+from app.plugins.builtins.automation.execution_engine import run_automation
+from app.plugins.builtins.automation.scheduler import (
+    calculate_next_run,
+    scheduler_tick,
+)
+from app.plugins.builtins.automation.agent_comm import send_agent_message
+
+
+class AutomationContract:
+    """Public API surface for the automation plugin."""
+
+    contract_name = "automation"
+
+    # ─── models ───
+    AgentDefinition = AgentDefinition
+    Automation = AutomationDefinition
+    CronJob = AutomationCronJob
+    AgentRun = AgentRun
+    AgentVersion = AgentVersion
+    AutomationRun = AutomationRun
+    AutomationVersion = AutomationVersion
+
+    # ─── services ───
+    AgentService = AgentService
+    AutomationService = AutomationService
+    CronJobService = CronJobService
+    RunLogService = RunLogService
+
+    # ─── agent_runner ───
+    run_agent = staticmethod(run_agent)
+
+    # ─── execution_engine ───
+    run_automation = staticmethod(run_automation)
+
+    # ─── scheduler ───
+    calculate_next_run = staticmethod(calculate_next_run)
+    scheduler_tick = staticmethod(scheduler_tick)
+
+    # ─── agent_comm ───
+    send_agent_message = staticmethod(send_agent_message)
+
+
+# ─── self-registration ───
+
+_contract = AutomationContract()
+get_contract_registry().register("automation", _contract)
+
+
+__all__ = [
+    "AutomationContract",
+    "AgentDefinition",
+    "Automation",
+    "CronJob",
+    "AgentService",
+    "AutomationService",
+    "CronJobService",
+    "RunLogService",
+]