Improve README: architecture diagram, filter DSL docs, Parquet schema, deployment guide

[ngoyal16]

Problem

The README.md is minimal and missing several important sections for a production Lambda project:

Missing:

1. Architecture diagram — No visual representation of the data pipeline (SQS → Lambda → S3 → KMS → Parquet → S3)
2. Deployment instructions — No Terraform/SAM/CDK examples or manual deployment steps
3. Filter configuration examples — The filters/ directory and filter query DSL (the not, selector, and, or, contains, dimensionExists node types in filter.go) are completely undocumented
4. Parquet schema documentation — The DatabaseActivityEvent struct in main.go defines 20+ fields but there is no schema reference
5. Input/Output format — No documentation of the expected S3 event format and the output Parquet structure
6. Troubleshooting — No common error scenarios and resolutions
7. Environment variable examples — No .env.example file

Proposed Solution

1. Add a Mermaid architecture diagram to README
2. Add a "Filter Configuration" section documenting the query DSL with examples
3. Add a "Deployment" section with at least manual steps
4. Add a "Parquet Schema" table documenting all fields in DatabaseActivityEvent
5. Create a .env.example file with all supported env vars
6. Add a "Troubleshooting" section

Acceptance Criteria

• README contains an architecture diagram (Mermaid)
• Filter query DSL is documented with at least one example per node type
• All 20+ Parquet fields are documented in a schema table
• .env.example file exists with all env vars
• Deployment instructions are present