[Audit Service] DB Config Priority and Doc Update (#419)

* Remove unused dotenv dependency and clean up test helper files

* feat(database): Enhance database configuration handling and add support for PostgreSQL

- Updated NewDatabaseConfig to prioritize database type and path settings.
- Default to in-memory SQLite if no configuration is provided.
- Added tests for various database configurations including in-memory, file-based SQLite, and PostgreSQL.
- Introduced comprehensive API documentation for audit log operations.
- Created architecture and database configuration documentation for better understanding of the service structure and setup.

* Nits to #419 (#420)

* Nits to #419

* Update audit-service/database/client.go

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

* Update audit-service/database/client.go

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

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Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

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Co-authored-by: ginaxu1 <167130561+ginaxu1@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

audit-service/.env.example

@@ -29,6 +29,8 @@ DB_TYPE=sqlite
 
 # Path to SQLite database file (default: ./data/audit.db)
 # The directory will be created automatically if it doesn't exist
+# For in-memory database (no persistence, testing only), use: DB_PATH=:memory:
+# For file-based database: DB_PATH=./data/audit.db
 DB_PATH=./data/audit.db
 
 # Maximum number of open database connections (default: 1 for SQLite)
@@ -88,3 +90,11 @@ DB_CONN_MAX_IDLE_TIME=15m
 #   - Multiple origins: CORS_ALLOWED_ORIGINS=http://localhost:5173,http://localhost:3000
 #   - Allow all: CORS_ALLOWED_ORIGINS=*
 CORS_ALLOWED_ORIGINS=http://localhost:5173
+
+# =============================================================================
+# Logging Configuration
+# =============================================================================
+
+# Log level: debug, info, warn, error (default: info)
+# Use "debug" for development, "info" or "warn" for production
+LOG_LEVEL=info