Add new skills for backend patterns, coding standards, continuous learning, and NestJS best practices

- Introduced backend patterns skill with guidelines on API design, database optimization, and server-side best practices.
- Added coding standards skill outlining universal coding principles for TypeScript, NestJS, and Node.js development.
- Implemented continuous learning skill to automatically extract reusable patterns from Cursor sessions.
- Created NestJS best practices skill detailing architecture patterns, dependency injection, error handling, and security measures.
- Included various rules and templates for NestJS best practices to ensure production-ready applications.

.cursor/rules/coding-style.mdc

@@ -0,0 +1,76 @@
+---
+description: Immutability, file organization, error handling, and input validation
+globs: "**/*.{ts,tsx,js,jsx}"
+alwaysApply: false
+---
+
+# Coding Style
+
+## Immutability (CRITICAL)
+
+ALWAYS create new objects, NEVER mutate:
+
+```javascript
+// WRONG: Mutation
+function updateUser(user, name) {
+  user.name = name  // MUTATION!
+  return user
+}
+
+// CORRECT: Immutability
+function updateUser(user, name) {
+  return {
+    ...user,
+    name
+  }
+}
+```
+
+## File Organization
+
+MANY SMALL FILES > FEW LARGE FILES:
+- High cohesion, low coupling
+- 200-400 lines typical, 800 max
+- Extract utilities from large components
+- Organize by feature/domain, not by type
+
+## Error Handling
+
+ALWAYS handle errors comprehensively:
+
+```typescript
+try {
+  const result = await riskyOperation()
+  return result
+} catch (error) {
+  console.error('Operation failed:', error)
+  throw new Error('Detailed user-friendly message')
+}
+```
+
+## Input Validation
+
+ALWAYS validate user input with class-validator DTOs (and Zod where DTOs are not used):
+
+```typescript
+import { z } from 'zod'
+
+const schema = z.object({
+  email: z.string().email(),
+  age: z.number().int().min(0).max(150)
+})
+
+const validated = schema.parse(input)
+```
+
+## Code Quality Checklist
+
+Before marking work complete:
+- [ ] Code is readable and well-named
+- [ ] Functions are small (<50 lines)
+- [ ] Files are focused (<800 lines)
+- [ ] No deep nesting (>4 levels)
+- [ ] Proper error handling
+- [ ] No console.log statements
+- [ ] No hardcoded values
+- [ ] No mutation (immutable patterns used)