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)

.golangci.yml

@@ -0,0 +1,29 @@
+version: "2"
+
+linters:
+  default: none
+  enable:
+    - errcheck
+    - staticcheck
+    - govet
+    - gosec
+    - ineffassign
+    - unused
+
+  settings:
+    errcheck:
+      check-type-assertions: false
+      check-blank: false
+
+  exclusions:
+    presets:
+      - std-error-handling
+      - common-false-positives
+    rules:
+      - path: _test\.go
+        linters:
+          - errcheck
+          - gosec
+
+run:
+  timeout: 5m