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>

src/lib/server/fleet/drift.ts

@@ -78,9 +78,11 @@ async function walkFiles(root: string): Promise<string[]> {
 
 /**
  * Scan every managed destination tree on a server (scripts, configs, plugins)
- * and return every file that isn't tracked in server_scripts. Beta artifacts
- * (-beta.gsc, .OLD, -beta.dll, composed dedicated_sp.cfg) are tracked while
- * a beta is active so they don't appear as drift.
+ * and return every file that isn't tracked in server_scripts. During a beta,
+ * the beta bytes live at the actual target filename and the inventory row is
+ * updated to reflect them, so beta files register as tracked with no
+ * special-casing (pre-beta bytes are snapshotted to beta_file_snapshots for
+ * rollback rather than left on disk as .OLD).
  *
  * The inventory table (server_scripts) keys on (server, rel_path) with no
  * destination discriminator. Rel_paths across destinations don't collide in