fix: resolve merge conflicts in anti-hallucination middleware and fallback manager

[ICholding]

Motivation

• Resolve remaining unresolved merge conflict fallout that caused a SyntaxError during deployment and restore merged functionality across verification and fallback services.
• Preserve and combine features from both incoming variants so repository verification, caching, memory integration, and robust fallback/circuit behavior work together.

Description

• Rebuilt backend/lib/anti-hallucination-middleware.js to include repository extraction, an in-memory TTL cache, optional Letta-style memory.get/memory.set integration, GitHub verification evidence, correction formatting, and backwards-compatible aliases (processResponse, validateResponse, apply).
• Rebuilt backend/services/fallback-manager.js as an EventEmitter-based manager adding circuit-breaker state tracking, provider-aware markSuccess/markFailure events, health monitoring via startHealthMonitor/stopHealthMonitor, and compatibility helpers executeWithFallback and executeWithHandlers plus getDefaultChains().
• Ensure both modules export the expected types and attach compatibility exports (module.exports = new FallbackManager(); module.exports.FallbackManager = FallbackManager; and module.exports = AntiHallucinationMiddleware;).
• Remove all merge markers and resolve syntax/shape inconsistencies between the two previous versions.

Testing

• Ran rg -n "^(<<<<<<<|=======|>>>>>>>)" -S backend/lib/anti-hallucination-middleware.js backend/services/fallback-manager.js to ensure no merge markers remain, which succeeded.
• Ran node --check backend/lib/anti-hallucination-middleware.js to verify syntax, which succeeded.
• Ran node --check backend/services/fallback-manager.js to verify syntax, which succeeded.

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