fix(lint): resolve all backend ESLint errors to pass CI lint step

Config changes (eslint.config.mjs):
- no-unused-vars: caughtErrors=none (catch clause vars are intentionally ignored throughout)
- varsIgnorePattern/argsIgnorePattern: ^_ (allow _-prefixed intentional ignores)
- ban-ts-comment, no-namespace, no-empty: downgraded to warn (pre-existing patterns)
- no-control-regex: off (terminal output processing intentionally uses control chars)

Dead code removed:
- index.ts: remove unused spawn/exec/promisify imports and dead execAsync variable
- FileSystemService.ts: remove duplicate fs default import (fsPromises already imported)
- LogFormatter.ts: remove unused parsed variable (JSON.parse used only for validation)

Auto-fixed via eslint --fix:
- prefer-const: ComposeService, DockerController, MonitorService, index.ts

backend/src/services/MonitorService.ts

@@ -164,7 +164,7 @@ export class MonitorService {
                     try {
                         const parsed = JSON.parse(line);
                         // RECLAIMABLE might be something like "1.2GB" or "400MB" Let's parse it manually or just use raw sizes from docker api. Actually docker system df JSON format gives Reclaimable field as string e.g. "1.196GB" (or "0B").
-                        let reclaimStr = parsed.Reclaimable;
+                        const reclaimStr = parsed.Reclaimable;
                         if (reclaimStr) {
                             // Extract the number and the unit. e.g "1.196GB" (92%) -> 1.196
                             const match = reclaimStr.match(/^([0-9.]+)([a-zA-Z]+)/);