fix: correct prompt admission, usage accounting and runtime state

[jkyberneees]

Attachment-only prompts were discarded, failed turns omitted paid token usage, model switches retained stale cost prices, and long-running scheduled jobs could overwrite newer scheduling state.

This change fixes all five confirmed functional regressions:

• Accept usable attachments without prompt text in WebSocket and REST requests, and reject empty placeholders before starting a turn.
• Resolve uploaded attachments against the effective current session when the request omits its session ID.
• Record provider usage for failed turns in sessions, server totals, and REST run snapshots.
• Re-resolve model cost prices from the original configuration when switching models, including fallback prices.
• Serialize scheduler projection/completion persistence, retain the current next-run time and cron signature, and avoid recreating state for removed jobs.

Validation: full changed-package race tests; targeted race tests covering the final amendments; golangci-lint v2.14.0; go vet; govulncheck. All passed. An independent adversarial sub-agent review found two gaps in the initial patch; both were fixed and regression-tested, and the final re-review found no remaining actionable issues.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	odek	88f397a	Commit Preview URL Branch Preview URL	Sep 30 2026, 07:28 AM