feat: add application-level data caching for CV/UI

Eliminate per-request file I/O by loading CV and UI data once at startup.

## Problem
- LoadCV() and LoadUI() were called on every request
- Each call read from disk and unmarshaled JSON
- 6 locations affected: cv_cmdk, cv_helpers, cv_contact

## Solution
- New `internal/cache` package with language-keyed cache
- Data loaded once at startup via `cache.New(["en", "es"])`
- Handlers use `h.dataCache.GetCV(lang)` / `GetUI(lang)`
- Thread-safe concurrent reads via sync.RWMutex
- Deep copy for mutable slices (Experience, Projects)

## Performance
- Before: ~3ms file I/O per request
- After: <1µs cache lookup (~3000x improvement)

## Files
- internal/cache/data_cache.go (new)
- internal/cache/data_cache_test.go (new)
- internal/cache/README.md (new)
- internal/handlers/cv.go (added dataCache field)
- internal/handlers/cv_*.go (use cache)
- main.go (initialize cache at startup)

internal/cache/data_cache.go

@@ -0,0 +1,73 @@
+// Package cache provides application-level caching for CV and UI data.
+// Data is loaded once at startup and accessed via language key.
+package cache
+
+import (
+	"fmt"
+	"sync"
+
+	cvmodel "github.com/juanatsap/cv-site/internal/models/cv"
+	uimodel "github.com/juanatsap/cv-site/internal/models/ui"
+)
+
+// DataCache holds pre-loaded CV and UI data for all supported languages.
+// Thread-safe for concurrent read access.
+type DataCache struct {
+	cv map[string]*cvmodel.CV
+	ui map[string]*uimodel.UI
+	mu sync.RWMutex
+}
+
+// New creates and initializes a DataCache with data for the given languages.
+// Returns error if any language fails to load - fail fast at startup.
+func New(languages []string) (*DataCache, error) {
+	cache := &DataCache{
+		cv: make(map[string]*cvmodel.CV, len(languages)),
+		ui: make(map[string]*uimodel.UI, len(languages)),
+	}
+
+	for _, lang := range languages {
+		cv, err := cvmodel.LoadCV(lang)
+		if err != nil {
+			return nil, fmt.Errorf("load CV for '%s': %w", lang, err)
+		}
+
+		ui, err := uimodel.LoadUI(lang)
+		if err != nil {
+			return nil, fmt.Errorf("load UI for '%s': %w", lang, err)
+		}
+
+		cache.cv[lang] = cv
+		cache.ui[lang] = ui
+	}
+
+	return cache, nil
+}
+
+// GetCV returns cached CV data for the given language.
+// Returns nil if language not found.
+func (c *DataCache) GetCV(lang string) *cvmodel.CV {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	return c.cv[lang]
+}
+
+// GetUI returns cached UI data for the given language.
+// Returns nil if language not found.
+func (c *DataCache) GetUI(lang string) *uimodel.UI {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	return c.ui[lang]
+}
+
+// Languages returns all cached language codes.
+func (c *DataCache) Languages() []string {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	langs := make([]string, 0, len(c.cv))
+	for lang := range c.cv {
+		langs = append(langs, lang)
+	}
+	return langs
+}