feat(server): production-grade modular structure with algorithms

[AnikethTS]

Restructured the flat main.py into a layered FastAPI app:

• core/config.py — pydantic-settings for type-safe env var loading
• core/prompts.py — system and summarization prompts
• models/chat.py — Message and ChatRequest Pydantic models
• middleware/ — sliding-window rate limiter (per IP, O(1) amortised)
• services/context — message preprocessing, greedy token-budget trimming,
  and midpoint summarization via a secondary Groq call
• services/streaming — SSE generator with exponential-backoff retry
• api/routes/ — chat and health endpoints as isolated routers
• main.py — create_app() factory wiring everything together