feat: Add field-level encryption for sensitive properties

[PieterjanDeClippel]

Summary

• Adds MintPlayer.Spark.Encryption.Abstractions project with [Encrypted] attribute and SparkEncryptionOptions (OwnKey + ModuleKeys)
• Adds MintPlayer.Spark.Encryption project with AES-256-GCM encrypt/decrypt, hooking into IDocumentStore.OnBeforeStore / OnAfterConversionToEntity events
• Encrypted values stored as opaque ENC:v1:{iv}:{ciphertext}:{tag} blobs — safe for RavenDB ETL replication between modules
• Cross-module key resolution via [Replicated(SourceModule)] attribute — consuming modules decrypt with the source module's key
• Startup validation throws if [Encrypted] is used on non-string properties
• Auto-generates a dev key in Development mode when OwnKey is not configured
• Demo: Fleet Car.VinNumber encrypted at rest, replicated as encrypted blob to HR, decrypted on read in both modules

Test plan

• dotnet build MintPlayer.Spark.sln compiles with no errors
• Temporarily add [Encrypted] to an int property → confirm startup throws InvalidOperationException
• Start Fleet app, create a Car with VinNumber → verify RavenDB document shows ENC:v1:... blob
• Load Car via Fleet API → confirm plaintext VinNumber is returned
• Start both Fleet + HR → create Car in Fleet → verify encrypted VinNumber blob replicates to HR database → load Car in HR → confirm decrypted VinNumber

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