feat(cli): add idempotent message delivery

[willgriffin]

Summary

• persist atomic channel-message idempotency receipts keyed by community, authenticated author, channel, and caller key digest
• make CLI retries return the original canonical event ID; expose --idempotency-key, replay JSON, and a 409 conflict exit
• preserve mention indexing on newly created idempotent messages and gate the Postgres regression in backend CI
• make Sprig’s existing headless buzz CLI release tag-bound and checksum/digest-pinnable

Validation

• PATH="$PWD/bin:$PATH" just ci
• isolated disposable Postgres proof: concurrent replay, changed-payload conflict, failed-write rollback/retry, and exact mention indexing
• targeted clippy, CLI/relay/migration tests, release-ref contract, formatting, and diff checks

Refs happyvertical/happyvertical.com#190, happyvertical/happyvertical.com#172, happyvertical/happyvertical.com#158

No release tag, OCI image, or artifact was published.

{"runtime":"codex","session":"codex-190-buzz-delivery-review-019fe85b","issue":"happyvertical/happyvertical.com#190","policy_revision":"1.0.0","head":"706437da8f6a3522c8f8a0ff7dfa098fda12fe40","validation":["PATH=\"$PWD/bin:$PATH\" just ci","cargo clippy -p buzz-db --all-targets -- -D warnings","disposable Postgres idempotency and mention-index contracts"]}

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 64bf18b235

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[willgriffin]

Follow-up validation at 706437da8f6a3522c8f8a0ff7dfa098fda12fe40 is clean.

• isolated Postgres: idempotency_receipt_is_atomic_under_concurrency_and_replays_canonical_event and idempotent_message_insert_populates_mentions both pass;
• just ci passes locally (Rust, desktop, web, and mobile);
• the P1 thread is resolved and an independent code review found no material findings.

Could a Block maintainer please review the current head and advise whether any additional changes are needed before merge? No release tag, OCI image, or artifact has been published.