WI-4: Release per-stream subscription connections on unsubscribe (#683)

[NickSeagull]

ADR open for review

This draft PR opens docs/decisions/0063-per-stream-subscription-connection-release.md (Status: Proposed) for review.

ADR-0063 fixes the per-stream subscription connection leak (WI-4 of epic #679): subscribeToStreamEventsImpl acquires a dedicated unpooled Hasql.acquire connection per stream subscription, but unsubscribeImpl never releases it, so under stream-subscription churn connections accumulate against the Azure Flexible Server B1ms ~35-usable ceiling.

The proposed fix ties each per-stream connection's lifetime to its SubscriptionId: SubscriptionInfo carries an onRemove cleanup Task (a no-op for global/entity subscriptions), the stream subscribe path registers Hasql.release of its connection, and removeSubscription runs that cleanup best-effort (logged, never blocking unsubscribe). This mirrors the ADR-0039 close/cleanup lifecycle one level down. The ADR also documents a concurrency cap of ~14 concurrent stream subscriptions on B1ms, derived from ADR-0060's 19 + N ≤ 35 budget (handed to WI-6, #685, for the deploy guide).

What lands later

The implementation (SubscriptionStore + Internal.hs changes, plus happy-path / error / boundary / no-leak regression tests) lands in subsequent commits on this branch through the feature pipeline. This PR stays a draft until the pipeline's PR gate (phase 16), at which point it is marked Ready for review.

Closes #683

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