feat(api): support anonymous confession threads

[BastiDood]

Adds first-class anonymous public thread support for confession channels, including /thread, Apps > Reply as Anonymous Thread, thread-aware confession posting, approval dispatch, resend behavior, durable attachment handling, and Discord thread creation error reporting. Resolves #27.

The PR also updates command registration and user docs so the new thread workflows, permissions, and attachment behavior are discoverable.

Implementation Notes

• Added a normalized thread destination model with pending_channel_thread and approved_channel_thread tables, plus confession.pending_channel_thread_id, so thread creation can be represented before and after moderator approval.
• Kept approval-required thread creation delayed until approval dispatch. Immediate non-approval submissions create the Discord thread during submission processing, while approval-required submissions persist the pending thread title and materialize the Discord thread later.
• Introduced shared channel-context resolution so commands can consistently distinguish root confession channels from existing Discord threads and route /confess, /thread, Reply Anonymously, and Reply as Anonymous Thread through the right channel/thread ids.
• Extended confession submission events with explicit modes: message, new-thread, and new-thread-reply. Empty modal state fields are parsed as null so modal custom ids can represent optional thread and parent-message context without sentinel objects.
• Added Discord API support for creating public threads directly from channels and from source messages. Thread creation failures now use known DiscordErrorCode values and are converted into friendly user-facing interaction responses.
• Kept the create-discord-thread step result as SerializedConfessionForProcess | string, using typeof result === 'string' to distinguish handled user-facing failures from successful prepared confession state.
• Split larger Inngest workflows into query/state helpers for submission, approval dispatch, and resend so BigInt-heavy database rows are serialized before crossing step boundaries.
• Reworked attachment handling around durable moderator-log uploads. Modal uploads are treated as ephemeral inputs, downloaded with size/content-length validation, uploaded into the moderator log, and later reused for approval, public posting, and resend.
• Added Discord CDN URL parsing for attachments and ephemeral-attachments, including URL normalization and metadata extraction from Discord message attachments or embed images.
• Updated moderator log rendering for thread destinations so logs show the parent channel and thread channel instead of treating all destinations as a single channel.
• Updated discord.json, docs, and command registration helpers for /thread and Reply as Anonymous Thread.
• Known follow-up: Failed anonymous thread creation still persists confession #42 tracks the remaining edge case where an immediate thread creation failure is reported to the user after the confession row has already been persisted.

Breaking Changes

None expected for users. A database migration is required to add thread-tracking tables and the nullable confession thread reference.

Test Cases

• /thread modal rejects existing-thread usage and missing public-thread/thread-message permissions.
• Reply as Anonymous Thread modal rejects recursive thread replies, target-channel mismatches, and missing permissions.
• /confess and anonymous reply modals resolve channel vs. thread destinations and enforce locked-thread permissions.
• Modal state parsing converts empty custom-id segments into null.
• Discord attachment CDN parsing accepts durable and ephemeral attachment URLs and rejects malformed/non-CDN URLs.
• Attachment download handling rejects bad responses, missing/invalid content lengths, and oversized downloads.
• Durable attachment metadata is extracted from Discord message attachments and image embeds.
• Thread creation Discord errors render Spectro-owned user-facing copy.
• Thread log payloads order parent/thread/reply fields predictably.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.