OPS-FIXES-1: AR-3 Data Protection Key-Persistenz + AR-4 AI-Env-Korrekte

AR-3 (P1): PersistKeysToFileSystem + persistentes Volume
  Program.cs: AddDataProtection().PersistKeysToFileSystem(keyRingPath).SetApplicationName(GerbilManager)
  Pfad konfigurierbar via DataProtection:KeyRingPath (env DataProtection__KeyRingPath);
  Fallback = ContentRoot/.data-protection-keys (Aspire-Dev-ephemeral, ok).
  compose.yaml: DataProtection__KeyRingPath: /data/keys + Volume-Mount keys:/data/keys.
  Volumes: neues 'keys' Volume (Bind-Mount auf NAS-Dataset KEYS_PATH=/mnt/SSD/gerbil/keys).
  .gitignore: .data-protection-keys/ ignoriert (Dev-only ephemeral keys).
  Verhindert: Gmail-App-Passwort wird nach Image-Redeploy unlesbar (bisher stilles inbox-fail).

AR-4 (P1): compose.yaml + .env.example: AI__* statt ANTHROPIC_API_KEY
  compose.yaml: ANTHROPIC_API_KEY entfernt (Code liest es nicht). Korrekte Vars:
    AI__BaseUrl: ${AI__BaseUrl:-}
    AI__ApiKey: ${AI__ApiKey:-}
    AI__Model: ${AI__Model:-gemini-2.0-flash}
  .env.example: AI__BaseUrl/ApiKey/Model + KEYS_PATH hinzugefuegt; ANTHROPIC_API_KEY entfernt.
  Quelle: docs/ai-provider.md (war korrekt, compose war falsch).
  Verhindert: alle 4 KI-Features (Verkaufstext, Inbox-Entwurf) blieben in prod stumm.

GATE: 139/139 C#-Tests, build gruen (using Microsoft.AspNetCore.DataProtection; framework-included).

.gitignore

@@ -131,6 +131,9 @@ $RECYCLE.BIN/
 # MemPalace per-project files (issue #185)
 mempalace.yaml
 entities.json
-
-# Runtime photo store (uploaded/imported gerbil photos) — never commit
-GerbilManagerWebAPI/photo-storage/
+
+# Runtime photo store (uploaded/imported gerbil photos) — never commit
+GerbilManagerWebAPI/photo-storage/
+
+# AR-3: Data Protection key ring (dev-only ephemeral keys) — never commit
+GerbilManagerWebAPI/.data-protection-keys/