#523 - Decompose auth middleware responsibilities

[brijesh-amin]

Parent: #523

Objective

Split authentication middleware into focused units to improve maintainability and reduce bootstrap/runtime risk.

In scope

• Separate JWT validation, user lookup/sync, role mapping, and permission resolution into composable functions
• Remove hard process termination from auth initialization path
• Keep existing auth outcomes and role assignment behavior

Acceptance criteria

• authmware orchestration delegates to smaller testable functions/modules
• Initialization failures propagate through startup error handling instead of process.exit in middleware
• Existing auth behavior remains compatible for idir/bceid role mapping
• Unit tests added/updated for extracted units

Notes

Prioritize readability and explicit error semantics over broad functional changes.