docs(db): align cache guide with runtime settings (#11018) (#11294)

Validated on a 3-PR combined board: 11018-database-cache-docs 1/1 within the board's 20/20 focused suite, typecheck:core clean, gates within baseline. DATABASE_GUIDE.md now cites the real 64 MiB runtime default and the 1..1,000,000 KiB range, locked with a regression test. Thank you @RaviTharuma!

changelog.d/maintenance/11018-database-cache-docs.md

@@ -0,0 +1 @@
+- **docs(database):** align the SQLite cache guide with the 65,536 KiB runtime default, supported 1–1,000,000 KiB range, and live Settings application behavior ([#11018](https://github.com/diegosouzapw/OmniRoute/issues/11018))

docs/ops/DATABASE_GUIDE.md

@@ -1,7 +1,7 @@
 ---
 title: "Database Schema & Operations Guide"
-version: 3.8.40
-lastUpdated: 2026-06-28
+version: 3.8.50
+lastUpdated: 2026-08-23
 ---
 
 # Database Schema & Operations Guide
@@ -43,12 +43,17 @@ For **single-user, single-instance** deployments (the primary OmniRoute use case
 db.pragma("journal_mode = WAL");
 db.pragma("busy_timeout = 2000");
 db.pragma("synchronous = NORMAL");
-// Settings > System & Storage > Cache Size is applied as KiB.
-db.pragma("cache_size = -16384");
+db.pragma(`cache_size = -${DEFAULT_DATABASE_SETTINGS.optimization.cacheSize}`);
 ```
 
 WAL allows **concurrent reads** during writes — important for the dashboard, which queries while requests are being recorded.
 
+The default cache size is **65,536 KiB (64 MiB)**. SQLite interprets a negative
+`cache_size` as an approximate upper bound in KiB and allocates pages on demand.
+**Settings > System & Storage > Cache Size** accepts integer values from **1 to
+1,000,000 KiB**; saving the setting applies it to the live database connection,
+and OmniRoute restores the persisted value at startup.
+
 ---
 
 ## Database Location

tests/unit/11018-database-cache-docs.test.ts

@@ -0,0 +1,15 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { DEFAULT_DATABASE_SETTINGS } from "../../src/types/databaseSettings.ts";
+
+const guide = readFileSync(new URL("../../docs/ops/DATABASE_GUIDE.md", import.meta.url), "utf8");
+
+test("database guide keeps cache tuning aligned with runtime settings (#11018)", () => {
+  const defaultCacheSize = DEFAULT_DATABASE_SETTINGS.optimization.cacheSize;
+
+  assert.match(guide, new RegExp(`${defaultCacheSize.toLocaleString("en-US")} KiB`));
+  assert.match(guide, /1 to\s+1,000,000 KiB/);
+  assert.match(guide, /saving the setting applies it to the live database connection/);
+  assert.match(guide, /restores the persisted value at startup/);
+});