Implement structured logging with ngx_log_error_core

Logging now uses nginx's native error log with proper levels:
- ERROR (level 3): Configuration/query/template failures
- WARN (level 4): Missing parameters
- INFO (level 6): Request processing, template resolution
- DEBUG (level 7): Detailed tracing

Log Format: [sqlite-serve:module] message

Example output:
- [info] [sqlite-serve:handler] Processing request for /books
- [info] [sqlite-serve:template] Resolved template: ./server_root/books/catalog.hbs
- [info] [sqlite-serve:params] Resolved 1 parameters
- [notice] [sqlite-serve:success] Rendered catalog.hbs with 1 params

Specialized logging functions:
- log_config_error(): Invalid configuration
- log_query_error(): SQL errors with query shown
- log_template_error(): Template failures with path
- log_param_error(): Parameter resolution issues
- log_request_success(): Successful processing info
- log_template_loading(): Template discovery

Uses ngx_log_error_core() C API directly for dynamic messages.

Test Coverage: 59 tests
Configuration: error_log set to debug level for visibility

conf/sqlite_serve.conf

@@ -8,7 +8,7 @@ events {
     worker_connections 1024;
 }
 
-error_log logs/error.log warn;
+error_log logs/error.log debug;
 
 http {
     # Global templates for shared components