Add a persistent delivery queue with retries

[sij411]

Features

• Queue outgoing ActivityPub deliveries
• Persist queued deliveries across restarts
• Retry temporary delivery failures with exponential backoff
• Track delivery status and attempt count
• Prevent duplicate delivery jobs
• Stop retrying permanent failures

Related

• crates/feder-server/src/send.rs
• crates/feder-server/src/storage/mod.rs
• crates/feder-server/src/storage/sqlite.rs