Raise notification on job completion and improve S3 wiring

[donaldgray]

Summary

• Adds SNS job completion notifications — publishes a message to a configurable topic ARN when a job completes or fails; no-ops when unconfigured (NullJobNotifier)
• Improves S3 service registration to use idiomatic AddAWSService<IAmazonS3>() with IOptions<S3TextStoreOptions> injection, removing the duplicate S3StorageOptions class
• AWS services (S3 and SNS) are now conditionally registered to avoid the SDK credential chain hanging in test environments
• Extracts service registrations from Program.cs into ServiceCollectionExtensions alongside the existing Hangfire extensions
• Adds dotnet format analyzers CI step to enforce editorconfig rules (e.g. csharp_prefer_braces) that the pre-commit hook misses
• Documents S3 storage configuration in docs/builder-api.md

Test plan

• Unit tests pass: dotnet test src/TextServices.sln
• CI format and analyzer checks pass
• With TextServices:Notifications:TopicArn unset, jobs complete without error (NullJobNotifier)
• With a valid SNS topic ARN configured, a notification is published on job completion and failure
• With TextServices:Storage:S3:BucketName set, S3TextStore is used; without it, FileSystemTextStore is used
• Tests do not hang (AWS credential chain not triggered without S3/SNS config)

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