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/date-time.mdc

@@ -0,0 +1,53 @@
+---
+description: All date-related data must use DateTime; never add parallel date files or helpers — extend the VO instead
+alwaysApply: true
+---
+
+# DateTime Value Object
+
+## Mandatory
+
+ALL data related to dates or date-times in the domain and application layers MUST use `DateTime` from `src/common/value-objects/date-time/`.
+
+- Construct from HTTP ISO via `DateTime.create(raw)`
+- Reconstruct from DB via `DateTime.fromUnixMs(ms)`
+- Compare with `equals()`, persist with `value` / `toJSON()` (unix **milliseconds**, UTC instant)
+- Render with `format()` / `toString()` using `DEFAULT_TIMEZONE` (default `GMT+7`) — timezone is for display and naive ISO interpretation only
+
+There is one date abstraction in this codebase: `DateTime`. Use it for every timestamp, occurrence, schedule instant, created/updated field, and any other date-related value in domain/application code.
+
+## Forbidden
+
+Do NOT:
+
+- Store or pass date-times as plain `string` / `number` / built-in `Date` in domain models, services, or repositories (beyond the DTO/HTTP or DB number boundary)
+- Create **any** new file or function for date parsing, validation, formatting, timezone conversion, or comparison
+- Add pipes, decorators, utils, helpers, or modules that bypass the VO
+- Use date libraries (`date-fns`, `luxon`, `moment`, `dayjs`, Temporal polyfills, etc.)
+
+```typescript
+// BAD — new helper / parallel logic
+function parseDate(raw: string): number { /* ... */ }
+function formatDate(ms: number): string { /* ... */ }
+user.occurredAt = Date.parse(dto.occurredAt)
+
+// GOOD — DateTime only
+const occurredAt = DateTime.create(dto.occurredAt)
+user.occurredAt = occurredAt // DateTime in domain
+await repo.save({ occurredAt: occurredAt.value }) // unix ms only at persistence edge
+```
+
+## When the VO is not enough
+
+If a requirement cannot be met with the current VO (e.g. new input formats, formatting options, comparisons, timezone forms):
+
+1. **Update** `src/common/value-objects/date-time/` (implementation + colocated tests) to match the requirement
+2. Do **not** create a new file, function, type, helper, or module for dates
+
+## Boundaries
+
+- HTTP DTOs may accept ISO `string`; map to `DateTime.create()` at the service boundary
+- Database columns may store unix milliseconds (`bigint` / integer); map to/from `DateTime` in the repository
+- Naive ISO strings (no `Z` / offset) are interpreted in `DEFAULT_TIMEZONE`
+- `InvalidDateTimeError` is the only date validation error; do not echo raw input in messages
+- Canonical storage is always UTC; never persist the display timezone as the instant