chore: add golangci-lint config and fix all lint issues

Add .golangci.yml enabling errcheck, staticcheck, govet, gosec,
ineffassign, and unused linters. Fix 66 issues across 16 files:
- Check all unchecked errors (errcheck)
- Use HTTP status constants instead of numeric literals (staticcheck)
- Replace deprecated LineUp/LineDown with ScrollUp/ScrollDown (staticcheck)
- Convert sprintf+write patterns to fmt.Fprintf (staticcheck)
- Add ReadHeaderTimeout to http.Server (gosec)
- Remove unused types and functions (unused)
- Add nolint comments for intentional patterns (InsecureSkipVerify,
  math/rand for jitter, dialect-only SQL formatting)

internal/tui/tab_nodes.go

@@ -2,8 +2,6 @@ package tui
 
 import (
 	"fmt"
-	"go-upkeep/internal/models"
-	"strings"
 	"time"
 )
 
@@ -71,26 +69,3 @@ func fmtNodeLastSeen(t time.Time) string {
 	}
 	return fmt.Sprintf("%dh ago", int(ago.Hours()))
 }
-
-func fmtProbeRegions(site models.Site, probeResults map[string]probeStatus) string {
-	if len(probeResults) == 0 {
-		return subtleStyle.Render("—")
-	}
-	var parts []string
-	for region, status := range probeResults {
-		short := region
-		if len(short) > 6 {
-			short = short[:6]
-		}
-		if status.isUp {
-			parts = append(parts, specialStyle.Render(short+":UP"))
-		} else {
-			parts = append(parts, dangerStyle.Render(short+":DN"))
-		}
-	}
-	return strings.Join(parts, " ")
-}
-
-type probeStatus struct {
-	isUp bool
-}