feat: per-tenant measurement_id (extend single-app to multi-tenant)

[intel352]

Per gocodealone-multisite SPEC.md V25 + T29.

Current plugin assumes single-app deployment with one Google Analytics measurement_id baked in config. Multisite host (gocodealone-multisite) needs per-tenant injection: each tenant declares its own ID in their content-repo's multisite.yaml, host injects only when set.

Scope

• Add per-tenant runtime configuration surface (vs single-app static config).
• Accept measurement_id from tenant context (passed by workflow-plugin-cms's analytics.injection module).
• ⊥ inject gtag for tenants without an ID.
• Privacy banner future-V (deferred).

Refs

• Multisite SPEC: https://github.com/GoCodeAlone/gocodealone-multisite/blob/main/SPEC.md
• Umbrella: GoCodeAlone/gocodealone-multisite#3
• CMS plugin analytics.injection module: GoCodeAlone/workflow-plugin-cms

[intel352]

Shipped via #5: anonymize_ip (build-time + per-call) + per-call tag_id documented. Multi-tenant hosts now have full parity with workflow-plugin-cms TenantConfig.