Replace -beta sidecar and .OLD backup with DB snapshots

Beta deploys used to rename foo.gsc -> foo.OLD and write beta bytes to
a foo-beta.gsc sidecar, forcing the mod loader to know the naming
convention. Now the beta content lives at the actual target filename
(same as configs already did) and the pre-beta bytes are captured in
a new beta_file_snapshots table for rollback.

- schema.sql: new beta_file_snapshots(beta_id, server, rel_path,
  prior_content BLOB, prior_sha256, prior_size). Handles scripts,
  plugins, and the composed dedicated_sp.cfg uniformly.
- db/index.ts: assertSnapshotSchemeSafe boot guard refuses to start
  if a legacy running/ready-promote beta is present on first upgrade.
  A settings sentinel (snapshots_v1) makes it one-shot.
- beta.ts openBeta: read prior bytes, insert snapshot, overwrite
  target directly.
- beta.ts rollbackBeta: restore from snapshot if had_original, else
  delete the fresh file. Drop snapshot rows after restore.
- beta.ts promoteBeta: no more rm -beta / rm .OLD calls, pull prior
  content from the snapshot table for the deployment audit, drop
  snapshots after promote.
- files.ts: delete betaFile() and oldFile().
- drift.ts: docstring updated; no special-casing needed since beta
  bytes and inventory now agree on the target rel_path.
- servers/[name]/+page.svelte: drop dead -beta/.OLD row filters.
- tests: plugin write asserts the target filename; unused
  betaFile/oldFile tests removed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/unit/files.test.ts

@@ -1,5 +1,4 @@
 import { describe, expect, it } from 'vitest';
-import { betaFile, oldFile } from '../../src/lib/server/fleet/files.js';
 import { slugify } from '../../src/lib/server/fleet/scripts.js';
 
 describe('slugify', () => {
@@ -9,20 +8,3 @@ describe('slugify', () => {
         expect(slugify('t5_kino_1')).toBe('t5-kino-1');
     });
 });
-
-describe('betaFile', () => {
-    it('inserts -beta before the extension', () => {
-        expect(betaFile('hud.gsc')).toBe('hud-beta.gsc');
-        expect(betaFile('perks/zom.gsc')).toBe('perks/zom-beta.gsc');
-    });
-    it('handles files without extension', () => {
-        expect(betaFile('README')).toBe('README-beta');
-    });
-});
-
-describe('oldFile', () => {
-    it('replaces extension with .OLD', () => {
-        expect(oldFile('hud.gsc')).toBe('hud.OLD');
-        expect(oldFile('perks/zom.gsc')).toBe('perks/zom.OLD');
-    });
-});