[Fix] #99 — Consolidate duplicate FastAPI app instances and fix metrics route

[adarsh-gautam-sys]

Summary

app/main.py created the FastAPI application instance multiple times, registered middlewares across different app instances, and defined the /metrics endpoint twice (once with authentication and once without it). Because Python executes the last definition, the unauthenticated version overrode the authenticated one, and middlewares registered to earlier app instances were lost. This PR consolidates the initialization into a single clean application structure and registers all middlewares and routers correctly.

Root Cause

Multiple FastAPI() instances and duplicate route definitions for /metrics in app/main.py caused routing conflicts, discarded middleware registrations, and security bypasses for the metrics endpoint.

Changes Made

• Consolidated FastAPI app initialization to a single app = FastAPI(...) call.
• Unified middleware configuration (CORS, Logging, Metrics) on the single app instance.
• Registered routers (auth, tenants, documents, rag) cleanly.
• Resolved incorrect/duplicate import paths for MetricsMiddleware, LoggingMiddleware, init_redis, and setup_tracing.
• Consolidated /metrics endpoint to a single definition requiring admin authentication.

Related Issue

Closes #99

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