add trustless IPFS gateway, use patched rclone, remove bitswap

[parkan]

remove the dead bitswap protocol and add an /ipfs/ endpoint implementing the trustless gateway spec via boxo/gateway

StorageBlockStore pools rclone handlers per storage and holds open a streaming file reader to serve sequential block reads efficiently -- DAG nodes return from the DB without storage I/O, and leaf blocks from the same file share a single held-open rclone stream

supports dag-scope=all/entity/block, path resolution, entity-bytes, format=car and format=raw. stale source files detected at read time are mapped to HTTP 409 via boxo's error status mechanism

[anjor]

I found 3 issues worth addressing in this PR:

1. handler/wallet/export_keys.go
migrateWalletAssignments skips unresolved legacy wallet_assignments rows, but still drops the table afterward. That makes the first export-keys run destructive for malformed keys, failed exports, or any actor that never gets a corresponding wallets row, because the preparation-wallet link is lost permanently. I think this should either fail when unresolved rows remain, or preserve the legacy table until every row has been migrated successfully.

2. store/storage_blockstore.go
   The cached s.active reader is shared across requests, but it was opened with the ctx from whichever request populated it. readerWithRetry stores that context and checks it on every read and reopen, so a later request can inherit a canceled or timed-out context from an earlier one and fail spuriously. Reusing the stream only within a single request, or decoupling the cached reader from request-scoped context, would avoid that.

3. store/storage_blockstore.go
   The mutex in readFileBlock is held across handler.Read, object validation, and the actual block read. That means one slow backend read serializes every /ipfs/ request through this process-wide blockstore. Under concurrent gateway traffic this looks like a single-reader bottleneck. I think the critical section should be narrowed to cache bookkeeping only, or moved to per-request state instead of a global lock.