Bug: Send to closed channel

I started seq-db in single mode with following flags:

• --total-size=64KB;
• --frac-size=32KB;

After that I wrote some logs and caught a panic with following message:

{"level":"info","ts":"2025-03-21T16:16:59.351+0300","message":"frac cache saved to disk","filepath":"/tmp/tmp.F1PfSOmYu9/.frac-cache","version":2}
panic: send on closed channel

goroutine 976 [running]:
gitlab.ozon.ru/sre/seq-db/frac.(*ActiveAppender).In(0xc0000d0888, 0xc0000d07e0, {0xc001e862a0, 0x9e, 0xd8}, {0xc002488000, 0xfa, 0x100}, 0xc00043e5c8, 0xc0000d0880)
        gitlab.ozon.ru/sre/seq-db/frac/active_appender.go:106 +0x3d0
gitlab.ozon.ru/sre/seq-db/frac.(*Active).Append(0xc0000d07e0, {0xc001e862a0, 0x9e, 0xd8}, {0xc002488000, 0xfa, 0x100}, 0xc00043e5c8)
        gitlab.ozon.ru/sre/seq-db/frac/active.go:333 +0xa5
gitlab.ozon.ru/sre/seq-db/fracmanager.(*FracManager).Append(0xc00043a000, {0xfde3a8, 0xc0024865d0}, {0xc001e862a0, 0x9e, 0xd8}, {0xc002488000, 0xfa, 0x100}, 0xc00043e5c8)
        gitlab.ozon.ru/sre/seq-db/fracmanager/frac_manager.go:357 +0x132
gitlab.ozon.ru/sre/seq-db/storeapi.(*GrpcV1).doBulk(0xc0000120e0, {0xfde3a8, 0xc0024865d0}, 0xc002481020)
        gitlab.ozon.ru/sre/seq-db/storeapi/grpc_bulk.go:53 +0x1b6
gitlab.ozon.ru/sre/seq-db/storeapi.(*GrpcV1).Bulk(0xc0000120e0, {0xfde450?, 0xc002157810?}, 0xc002481020)
        gitlab.ozon.ru/sre/seq-db/storeapi/grpc_bulk.go:26 +0x1a5
gitlab.ozon.ru/sre/seq-db/storeapi.inMemoryAPIClient.Bulk({0xc002143380?}, {0xfde450?, 0xc002157810?}, 0xc002481020?, {0x20?, 0xd20600?, 0x1?})
        gitlab.ozon.ru/sre/seq-db/storeapi/client.go:26 +0x111
gitlab.ozon.ru/sre/seq-db/proxy/bulk.sendBulkToHost({0xfde450, 0xc002157810}, {{0xe98fdf?, 0xc00029b7b8?}, {0xfde878?, 0xc00041a868?}}, 0xc002481020)
        gitlab.ozon.ru/sre/seq-db/proxy/bulk/seqdb_client.go:195 +0x122
gitlab.ozon.ru/sre/seq-db/proxy/bulk.(*SeqDBClient).sendBulkToStores.(*shard).Bulk.func1.1()
        gitlab.ozon.ru/sre/seq-db/proxy/bulk/seqdb_client.go:178 +0x69
created by gitlab.ozon.ru/sre/seq-db/proxy/bulk.(*SeqDBClient).sendBulkToStores.(*shard).Bulk.func1 in goroutine 921
        gitlab.ozon.ru/sre/seq-db/proxy/bulk/seqdb_client.go:177 +0x274
make: *** [Makefile:34: run] Error 2

I haven't tested such flags when proxy and store are running, but I believe the bug will remain.

Start with frac/active_appender.go:106 and trace the close and send lifecycle through frac/active.go:333. Reproduce in single mode with --total-size=64KB and --frac-size=32KB, then exercise the log-writing path shown in the report. Done means the workload no longer panics with "send on closed channel".