feat(backend): answer conversation queries from repository

[sandersonstabo]

Depends on #322.

Scope

• answer bounded ConversationRequest::Query through the accepted durable snapshot reader
• preserve exact frame correlation and existing repository failure classification
• keep Subscribe/Unsubscribe honestly unbacked until the per-connection registrar exists
• add success, unknown-thread, subscription-control, and no-origin integration coverage

Verification

• centralized jobs=1 backend leaf gate passed at exact ecaa252909c3c0445f62ec916c204e6e5ad47ceb
• //tests/backend:request_handler_test passed (32 cases)
• fresh backend and request-handler Clippy markers
• root format, locked/offline Cargo metadata, exact path/hash/status guards, Bazel shutdown 0

Receipt: D:/artisan-editor-gate-logs/backend-conversation-query-20260829-gate1/receipt.json

Excluded / next

Per-connection subscription registrar, activation-after-response ordering, durable replay delivery, notifier/writer ownership, startup custody, and frontend projection remain later packets.

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Summary by cubic

Answers bounded conversation queries directly from the durable repository snapshot instead of failing all conversation requests as unbacked. Queries now return a snapshot or a typed ThreadUnknown failure; subscribe/unsubscribe remain unbacked until the per-connection registrar exists.

• Query reads never consult the admission origin and preserve request correlation.
• Unknown threads map to ThreadUnknown; corrupt or invariant state maps to non-retryable Internal; database operation failures remain retryable Internal.
• Subscribe and unsubscribe keep failing with a non-retryable Internal error that names the missing subscription capability.
• Adds integration coverage for snapshot success, unknown-thread, subscription control, and no-origin queries.

^{Written for commit ecaa252. Summary will update on new commits.}