security: P0 fixes - SQL injection prevention + data race fix

- Add identifier validation for database names in PostgreSQL and MySQL
  - validateIdentifier() rejects names with invalid characters
  - quoteIdentifier() safely quotes identifiers with proper escaping
  - Max length: 63 chars (PostgreSQL), 64 chars (MySQL)
  - Only allows alphanumeric + underscores, must start with letter/underscore

- Fix data race in notification manager
  - Multiple goroutines were appending to shared error slice
  - Added errMu sync.Mutex to protect concurrent error collection

- Security improvements prevent:
  - SQL injection via malicious database names
  - CREATE DATABASE `foo`; DROP DATABASE production; --`
  - Race conditions causing lost or corrupted error data

CHANGELOG.md

@@ -23,6 +23,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Provides actionable error messages with root cause
 
 ### Fixed
+- **P0: SQL Injection** - Added identifier validation for database names in CREATE/DROP DATABASE to prevent SQL injection attacks; uses safe quoting and regex validation (alphanumeric + underscore only)
+- **P0: Data Race** - Fixed concurrent goroutines appending to shared error slice in notification manager; now uses mutex synchronization
 - **P0: psql ON_ERROR_STOP** - Added `-v ON_ERROR_STOP=1` to psql commands to fail fast on first error instead of accumulating millions of errors
 - **P1: Pipe deadlock** - Fixed streaming compression deadlock when pg_dump blocks on full pipe buffer; now uses goroutine with proper context timeout handling
 - **P1: SIGPIPE handling** - Detect exit code 141 (broken pipe) and report compressor failure as root cause