Use async subprocess for ledger anchoring (event loop blocking)

[glandua]

Summary

ledger_anchor.py:broadcast_anchor() is declared async def but calls subprocess.run() (synchronous) and time.sleep(5) in a polling loop. This blocks the entire event loop for up to 30 seconds during anchoring.

Current Code

async def broadcast_anchor(claim_rid: str, content_hash: str) -> dict:
    # ...
    tx_result = subprocess.run([regen_bin, ...], ...)  # blocks event loop
    # ...
    for attempt in range(6):
        time.sleep(5)  # blocks event loop for 5s per attempt
        query_result = subprocess.run(...)  # blocks again

Proposed Fix

Option A (preferred): Use asyncio.create_subprocess_exec() + asyncio.sleep():

proc = await asyncio.create_subprocess_exec(
    regen_bin, "tx", "data", "anchor", iri, ...,
    stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE
)
stdout, stderr = await proc.communicate()

Option B (simpler): Run blocking code in thread executor:

loop = asyncio.get_event_loop()
result = await loop.run_in_executor(None, _blocking_broadcast, ...)

Impact

Other API endpoints remain responsive during anchoring operations. No API contract change.