Identify and fix performance issues across services

[Copilot]

Analyzed codebase for performance bottlenecks and inefficient patterns. Created comprehensive documentation and implemented critical fixes.

Documentation

Added docs/PERFORMANCE_IMPROVEMENTS.md covering:

• 10 identified issues ranked by priority with suggested fixes
• Code patterns for future improvements (sync.Pool, batch I/O, context propagation)
• Estimated impact analysis

Code Fixes

MetricsCollector double-locking → atomic operations

Replaced nested mutex locks with sync/atomic for counters and gauges. Uses read-lock fast path with double-check locking for map access.

// Before: O(2) lock acquisitions per operation
m.mu.Lock()
counter.mu.Lock()
counter.value++

// After: O(1) lock on fast path, atomic increment
m.mu.RLock()
counter, exists := m.counters[name]
m.mu.RUnlock()
if exists {
    atomic.AddInt64(&counter.value, 1)
    return
}

String concatenation O(n²) → O(n)

• story_service.go: Replaced += in loop with strings.Builder
• formatCharacters(): Added pre-allocation with documented capacity constant

Redundant operations removed

• context_service.go: Removed cache update immediately before invalidation
• item_service.go: Fixed duplicate delete() call in cache invalidation

Original prompt

Identify and suggest improvements to slow or inefficient code

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