fix(dbmanager): keep the pool alive across errors and restarts

Implements the fixes from audit/pkg/dbmanager.audit.md.

- Stop closing the shared *sql.DB to recover from errors. Adapter
  factories and the health checker no longer call Reconnect; Reconnect is
  atomic and operator-only.
- Postgres uses a custom driver.Connector: Reconnect retires pooled
  connections by generation without closing the pool, so held Bun/GORM
  handles keep working. Verified against a live server restart.
- Add TCP keepalive, TCP_USER_TIMEOUT, a bounded reuse ping and
  statement_timeout as a runtime parameter; drop the 2 min timeout floor.
- Health check pings without holding the connection lock.
- Listener: single goroutine pair, bounded Close without UNLISTEN, and
  serialised use of the pgx connection (fixes conn busy and a close race).
- Fix Connect/Close/Connect/Close panic, idempotent Connect, dial outside
  the manager lock, clean up on partial failure.
- SQLite: pin :memory: to one connection, pragmas via DSN.
- Escape credentials in Postgres/MSSQL/Mongo DSNs; sslmode defaults to
  prefer. Wire retry settings, publish metrics, fix logger calls.
- NewConnectionFromDB: Close is a no-op with a warning (caller owns the
  pool); Reconnect only pings.
- Document correct usage in the README; mark the audit with what was done.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

pkg/dbmanager/providers/existing_db.go

@@ -7,6 +7,8 @@ import (
 	"sync"
 
 	"go.mongodb.org/mongo-driver/mongo"
+
+	"github.com/bitechdev/ResolveSpec/pkg/logger"
 )
 
 // ExistingDBProvider wraps an existing *sql.DB connection
@@ -44,16 +46,27 @@ func (p *ExistingDBProvider) Connect(ctx context.Context, cfg ConnectionConfig)
 	return nil
 }
 
-// Close closes the underlying database connection
-func (p *ExistingDBProvider) Close() error {
-	p.mu.Lock()
-	defer p.mu.Unlock()
+// Refresh verifies the wrapped database is still reachable. The pool belongs to
+// the caller and cannot be re-dialed here, so it is never closed to "reconnect".
+func (p *ExistingDBProvider) Refresh(ctx context.Context) error {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
 
 	if p.db == nil {
-		return nil
+		return fmt.Errorf("database connection is nil")
 	}
+	return p.db.PingContext(ctx)
+}
 
-	return p.db.Close()
+// OwnsDB reports whether Close releases the wrapped database. It never does:
+// the *sql.DB was opened by the caller, who is responsible for closing it.
+func (p *ExistingDBProvider) OwnsDB() bool { return false }
+
+// Close is a no-op for the wrapped database. The pool belongs to the caller, so
+// closing it here would break the caller's other users of it.
+func (p *ExistingDBProvider) Close() error {
+	logger.Warn("Not closing externally provided database: name=%s; the caller owns this *sql.DB and must close it", p.name)
+	return nil
 }
 
 // HealthCheck verifies the connection is alive