feat(content): Add network subscription scheduler

[leumor]

Summary

• Add Platform API contract v8 content subscription endpoints and the content.subscribe capability for app-scoped USK subscriptions.
• Add durable subscription metadata models, file/in-memory stores, pressure gating, and a conservative deterministic scheduler with limits, backoff, dedupe, and safe summaries.
• Wire the scheduler through HTTP runtime/app-platform services and update Feed Reader plus the browser SDK to use subscription helpers.
• Update docs, permission rationale, release-certification evidence, smoke checks, and focused tests for subscription routing, store, scheduler, SDK, and reference app behavior.

Test Plan

• ./gradlew spotlessApply
• ./gradlew :platform-api:test :platform-sdk-js:test :apps:feed-reader:test :bridge-http-runtime:test :adapter-http-legacy-admin:test
• python3 tools/release-certification/app_platform_smoke.py --self-test
• python3 tools/release-certification/app_platform_docs_check.py
• git diff --check --cached

Notes

• Full ./gradlew test was not run locally for this PR-sized change set.

[sonarqubecloud]

Quality Gate failed

Failed conditions
77.6% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud