fix: remove event-loop-blocking cold start; unify profiler to cursor-based incremental

Cold start fetched all logs in one bulk query then processed them in a tight
synchronous loop with no yields, blocking the asyncio event loop for seconds
on datasets of 30K+ rows. This stalled every concurrent await — including the
SSE stream generator's initial DB calls — causing the dashboard to show
INITIALIZING SENSORS indefinitely.

Changes:
- Drop _cold_start() and get_all_logs_raw(); uninitialized state now runs the
  same cursor loop as incremental, starting from last_log_id=0
- Yield to the event loop after every _BATCH_SIZE rows (asyncio.sleep(0))
- Add SSE keepalive comment as first yield so the connection flushes before
  any DB work begins
- Add Cache-Control/X-Accel-Buffering headers to StreamingResponse

tests/test_base_repo.py

@@ -21,7 +21,6 @@ class DummyRepo(BaseRepository):
     async def get_total_bounties(self, **kw): await super().get_total_bounties(**kw)
     async def get_state(self, k): await super().get_state(k)
     async def set_state(self, k, v): await super().set_state(k, v)
-    async def get_all_logs_raw(self): await super().get_all_logs_raw()
     async def get_max_log_id(self): await super().get_max_log_id()
     async def get_logs_after_id(self, last_id, limit=500): await super().get_logs_after_id(last_id, limit)
     async def get_all_bounties_by_ip(self): await super().get_all_bounties_by_ip()
@@ -58,7 +57,6 @@ async def test_base_repo_coverage():
     await dr.get_total_bounties()
     await dr.get_state("k")
     await dr.set_state("k", "v")
-    await dr.get_all_logs_raw()
     await dr.get_max_log_id()
     await dr.get_logs_after_id(0)
     await dr.get_all_bounties_by_ip()