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/mail/services.py

@@ -1545,6 +1545,19 @@ async def send_mail_via_smtp(
             filename=filename,
         )
 
+    # ── Hook: mail.before_send (Filter) ──
+    from app.core.hooks import apply_filters
+    mail_data = {
+        "subject": subject,
+        "body_html": body_html,
+        "body_text": body_text,
+        "to_addrs": to_addrs,
+        "cc_addrs": cc_addrs,
+        "bcc_addrs": bcc_addrs,
+        "attachment_paths": attachment_paths,
+    }
+    mail_data = await apply_filters("mail.before_send", mail_data)
+
     # Send via SMTP
     password = await get_account_password(account)
     try:
@@ -1559,6 +1572,10 @@ async def send_mail_via_smtp(
         await smtp.send_message(msg, recipients=recipients)
         await smtp.quit()
 
+        # ── Hook: mail.after_send (Action) ──
+        from app.core.hooks import do_action
+        await do_action("mail.after_send", mail_data, db=db, account=account, msg_id=msg_id)
+
         # Store sent mail in Sent folder — use configured mapping if set,
         # otherwise flexible lookup to handle different IMAP naming conventions
         if account.sent_folder_imap_name: