feat(server): Implement REST API Routers for Session & Stats Retrieval (Sub-task 3.3)

[archittmittal]

Description

Implement the core web HTTP endpoints for historical session queries.
Part of parent issue: #3

Tasks

• Create paginated GET /sessions router endpoint.
• Create /sessions/{id}, /stats, and /sessions/{id}/events filter routers.

Acceptance Criteria

• Response times under normal loads execute in less than 200ms.
• Payloads follow schema definitions in SCHEMA.md exactly.