Add channel subscriptions, counts, creators, and DB query indexes

[king-aj-the-first]

Summary

Adds on-chain notification channel subscriptions (creator metadata, subscriber counts, batch subscribe) and listener DB query-performance indexes.

Task 1 — Subscription Count View

• New read-only get_subscriber_count(channel_id) on the contract
• Count maintained on subscribe / unsubscribe (and reflected in get_channel)
• Tests cover increment, decrement, and duplicate-subscribe leaving count unchanged

Task 2 — Batch Subscription Functionality

• New batch_subscribe(channel_ids, subscriber) processes multiple channels in one tx
• Structural errors (empty / >50 / paused) abort before mutation
• Per-channel failures (missing, inactive, already subscribed) are skipped — successful subscriptions keep valid state; failures do not corrupt prior state
• Returns BatchSubscribeResult { succeeded, failed, subscribed_ids }
• Gas notes documented in docs/BATCH_SUBSCRIBE_GAS.md

Task 3 — Channel Creator Information

• create_channel permanently stores the creator wallet on NotificationChannel
• Queryable via get_channel_creator(id) and get_channel(id).creator
• Tests verify storage and NotFound for unknown channels

Task 4 — Database Query Performance

• Identified slow paths: scheduler claim, post-claim lock fetch, notification search, processed-event search, metrics joins, rate-limit audit
• Added composite/partial indexes in schema.sql + migration 002-query-performance-indexes
• Documented plans, measurement steps, and expected improvements in docs/DATABASE_QUERY_PERFORMANCE.md
• Added query-performance.test.ts to assert indexes exist and claim queries use an indexed plan

New contract APIs

Function	Purpose
create_channel	Create channel + store creator
get_channel	Full metadata
get_channel_creator	Creator address
get_subscriber_count	Active subscriber count
is_channel_subscriber	Membership check
subscribe / unsubscribe	Single-channel ops
batch_subscribe	Multi-channel subscribe

Test plan

• Contract: channel_subscription_test (creator, count, batch + partial failure)
• Listener: query-performance.test.ts
• Review docs/BATCH_SUBSCRIBE_GAS.md and docs/DATABASE_QUERY_PERFORMANCE.md
• Apply migration 002 on an existing listener DB and confirm indexes via PRAGMA index_list

closes #471
closes #478
closes #480
closes #486

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