Add MCP project sync staged upload API

[antonchen]

Summary

• add staged project sync job endpoints for MCP uploads
• enforce declared byte admission limits and backlog budgeting with retry hints
• harden replace workflow with snapshot rollback, recovery, and postflight index/metadata repair

Validation

• cargo fmt --all --manifest-path /tmp/next-plaid-pr-project-sync/Cargo.toml -- --check
• cargo clippy --manifest-path /tmp/next-plaid-pr-project-sync/Cargo.toml -p next-plaid-api --all-targets --features "openblas model" -- -D warnings
• cargo test --manifest-path /tmp/next-plaid-pr-project-sync/Cargo.toml -p next-plaid-api --lib --features model project_sync -- --nocapture
• cargo test --manifest-path /tmp/next-plaid-pr-project-sync/Cargo.toml -p next-plaid-api --test integration_tests --features model test_project_sync_create_upload_cancel_status -- --nocapture
• TMPDIR=/home/anton/.cache/next-plaid-pr-project-sync-tmp CARGO_TARGET_DIR=/home/anton/.cache/next-plaid-pr-project-sync-target cargo test --manifest-path /tmp/next-plaid-pr-project-sync/Cargo.toml -p next-plaid-api --test integration_tests test_project_sync_admission_limits -- --nocapture
• TMPDIR=/home/anton/.cache/next-plaid-pr-project-sync-tmp CARGO_TARGET_DIR=/home/anton/.cache/next-plaid-pr-project-sync-target cargo test --manifest-path /tmp/next-plaid-pr-project-sync/Cargo.toml -p next-plaid-api --test integration_tests test_project_sync_finalize_reaches_terminal_status_without_model -- --nocapture

Implemented entirely by Codex.