Closes#921
## Summary
Follow-up to #920 (incremental auto-vacuum). Addresses both items from
the adversarial review:
### 1. RW connection caching
Previously, every call to `openRW(dbPath)` opened a new SQLite RW
connection and closed it after use. This happened in:
- `runIncrementalVacuum` (~4x/hour)
- `PruneOldPackets`, `PruneOldMetrics`, `RemoveStaleObservers`
- `buildAndPersistEdges`, `PruneNeighborEdges`
- All neighbor persist operations
Now a single `*sql.DB` handle (with `MaxOpenConns(1)`) is cached
process-wide via `cachedRW(dbPath)`. The underlying connection pool
manages serialization. The original `openRW()` function is retained for
one-shot test usage.
### 2. DBConfig dedup
`DBConfig` was defined identically in both `cmd/server/config.go` and
`cmd/ingestor/config.go`. Extracted to `internal/dbconfig/` as a shared
package; both binaries now use a type alias (`type DBConfig =
dbconfig.DBConfig`).
## Tests added
| Test | File |
|------|------|
| `TestCachedRW_ReturnsSameHandle` | `cmd/server/rw_cache_test.go` |
| `TestCachedRW_100Calls_SingleConnection` |
`cmd/server/rw_cache_test.go` |
| `TestGetIncrementalVacuumPages_Default` |
`internal/dbconfig/dbconfig_test.go` |
| `TestGetIncrementalVacuumPages_Configured` |
`internal/dbconfig/dbconfig_test.go` |
## Verification
```
ok github.com/corescope/server 20.069s
ok github.com/corescope/ingestor 47.117s
ok github.com/meshcore-analyzer/dbconfig 0.003s
```
Both binaries build cleanly. 100 sequential `cachedRW()` calls return
the same handle with exactly 1 entry in the cache map.
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Co-authored-by: you <you@example.com>
Kpa-clawbot