Admin Decomposition: Consolidate 18 handler modules into 2 generic handlers (Phase A)

[intel352]

Overview

The admin config has 18 intermediate api.query/api.command module definitions (9 domain pairs) that each delegate to a Go service. Consolidate to just 2 generic handlers.

Reference: docs/ADMIN_CONFIG_DECOMPOSITION.md — Phase A
Template: PR #56, commit ab64254 (feature flag migration pattern)

Problem

• 16 unnecessary handler modules exist solely to name a delegate target
• QueryHandler and CommandHandler work fine without delegate config — it's optional
• When a route has a pipeline, the pipeline runs; the delegate is only a fallback

Tasks

Remove 16 modules from admin/config.yaml:

admin-engine-queries, admin-engine-commands, admin-schema-queries, admin-ai-queries, admin-ai-commands, admin-component-queries, admin-component-commands, admin-timeline-queries, admin-replay-queries, admin-replay-commands, admin-dlq-queries, admin-dlq-commands, admin-backfill-queries, admin-backfill-commands, admin-billing-queries, admin-billing-commands

Rename + simplify 2 remaining handlers:

• admin-v1-queries → admin-queries (type: api.query, no delegate)
• admin-v1-commands → admin-commands (type: api.command, no delegate)

Update all ~77 route handler: references:

• All 18 old handler names → one of admin-queries or admin-commands

Verification

• go build -o server ./cmd/server compiles
• ./server -config admin/config.yaml starts
• All delegate routes still work (delegates still registered as services)
• go test ./... passes

Files to Modify

• admin/config.yaml — module removals + route handler rewiring