fix: enable SQLite incremental auto-vacuum so DB shrinks after retention (#919) (#920)

Closes #919

## Summary

Enables SQLite incremental auto-vacuum so the database file actually
shrinks after retention reaper deletes old data. Previously, `DELETE`
operations freed pages internally but never returned disk space to the
OS.

## Changes

### 1. Auto-vacuum on new databases
- `PRAGMA auto_vacuum = INCREMENTAL` set via DSN pragma before
`journal_mode(WAL)` in the ingestor's `OpenStoreWithInterval`
- Must be set before any tables are created; DSN ordering ensures this

### 2. Post-reaper incremental vacuum
- `PRAGMA incremental_vacuum(N)` runs after every retention reaper cycle
(packets, metrics, observers, neighbor edges)
- N defaults to 1024 pages, configurable via `db.incrementalVacuumPages`
- Noop on `auto_vacuum=NONE` databases (safe before migration)
- Added to both server and ingestor

### 3. Opt-in full VACUUM for existing databases
- Startup check logs a clear warning if `auto_vacuum != INCREMENTAL`
- `db.vacuumOnStartup: true` config triggers one-time `PRAGMA
auto_vacuum = INCREMENTAL; VACUUM`
- Logs start/end time for operator visibility

### 4. Documentation
- `docs/user-guide/configuration.md`: retention section notes that
lowering retention doesn't immediately shrink the DB
- `docs/user-guide/database.md`: new guide covering WAL, auto-vacuum,
migration, manual VACUUM

### 5. Tests
- `TestNewDBHasIncrementalAutoVacuum` — fresh DB gets `auto_vacuum=2`
- `TestExistingDBHasAutoVacuumNone` — old DB stays at `auto_vacuum=0`
- `TestVacuumOnStartupMigratesDB` — full VACUUM sets `auto_vacuum=2`
- `TestIncrementalVacuumReducesFreelist` — DELETE + vacuum shrinks
freelist
- `TestCheckAutoVacuumLogs` — handles both modes without panic
- `TestConfigIncrementalVacuumPages` — config defaults and overrides

## Migration path for existing databases

1. On startup, CoreScope logs: `[db] auto_vacuum=NONE — DB needs
one-time VACUUM...`
2. Set `db.vacuumOnStartup: true` in config.json
3. Restart — VACUUM runs (blocks startup, minutes on large DBs)
4. Remove `vacuumOnStartup` after migration

## Test results

```
ok  github.com/corescope/server    19.448s
ok  github.com/corescope/ingestor  30.682s
```

---------

Co-authored-by: you <you@example.com>

config.example.json

@@ -9,6 +9,11 @@
     "packetDays": 30,
     "_comment": "nodeDays: nodes not seen in N days moved to inactive_nodes (default 7). observerDays: observers not sending data in N days are removed (-1 = keep forever, default 14). packetDays: transmissions older than N days are deleted (0 = disabled)."
   },
+  "db": {
+    "vacuumOnStartup": false,
+    "incrementalVacuumPages": 1024,
+    "_comment": "vacuumOnStartup: run one-time full VACUUM to enable incremental auto-vacuum on existing DBs (blocks startup for minutes on large DBs; requires 2x DB file size in free disk space). incrementalVacuumPages: free pages returned to OS after each retention reaper cycle (default 1024). See #919."
+  },
   "https": {
     "cert": "/path/to/cert.pem",
     "key": "/path/to/key.pem",