feat: structured admin_action log channel

[Bilalishaq7]

closes #561

Description

Implements a dedicated, isolated logging channel for security and administrative event tracking as part of the audit compliance initiative. This PR replaces generic console routing inside administrative routes with a structured, high-context Pino child logger tagged with the channel identity admin_action.

Changes

• New Middleware (src/middleware/adminLog.ts): Extracted a dedicated child logger instance from the root Pino configuration containing the permanent context flag { channel: 'admin_action' }. Built a lifecycle-aware Express hook tracking route metrics (method, full path, statusCode, durationMs, ip, and userAgent).
• Identity Context Integration: Correlates actions directly back to the authenticated system operator using res.locals.adminActor populated by the upstream adminAuth cycle.
• Global Router Attachment (src/routes/admin.ts): Mounted the interceptor at the root administrative route controller, automatically cascading structural monitoring across all present and future nested modules (/apis, /webhooks, /usage, etc.) with zero individual controller overhead.
• Unit Testing (src/middleware/adminLog.test.ts): Introduced a mock lifecycle environment validating precise execution parameters, response timing traps, and contextual metadata accuracy, ensuring >90% line coverage constraints are explicitly met.
• Documentation (README.md): Updated codebase reference guides with operational notices tracking the isolated admin_action stream profile for external aggregation profiles (e.g., Datadog, Logstash).

Verification Results

• Error catalog updates fully synchronized via script engines.
• Local test runners validating all baseline structures without runtime anomalies.