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/db/schema.sql

@@ -123,8 +123,9 @@ CREATE INDEX IF NOT EXISTS idx_beta_scripts_beta ON beta_scripts(beta_id);
 CREATE INDEX IF NOT EXISTS idx_beta_scripts_script ON beta_scripts(script_id);
 
 -- Per (beta, server, script) prior-file state. A server row in beta_servers
--- covers restart progress; this table covers what each script's target
--- looked like on that server before the beta wrote its -beta.gsc.
+-- covers restart progress; this table covers whether each script's target
+-- existed on that server before the beta overwrote it — snapshot content
+-- lives in beta_file_snapshots.
 CREATE TABLE IF NOT EXISTS beta_server_scripts (
     beta_id           INTEGER NOT NULL REFERENCES betas(id) ON DELETE CASCADE,
     server            TEXT NOT NULL REFERENCES servers(name),
@@ -135,6 +136,21 @@ CREATE TABLE IF NOT EXISTS beta_server_scripts (
 );
 CREATE INDEX IF NOT EXISTS idx_beta_server_scripts_beta ON beta_server_scripts(beta_id);
 
+-- Pre-beta bytes of every target file the beta overwrote on each server.
+-- Absence of a row for (beta, server, rel_path) means the file did not exist
+-- before the beta wrote it — rollback deletes the file rather than restoring.
+-- Handles scripts, plugins, and the composed dedicated_sp.cfg uniformly.
+CREATE TABLE IF NOT EXISTS beta_file_snapshots (
+    beta_id       INTEGER NOT NULL REFERENCES betas(id) ON DELETE CASCADE,
+    server        TEXT NOT NULL REFERENCES servers(name) ON DELETE CASCADE,
+    rel_path      TEXT NOT NULL,
+    prior_content BLOB NOT NULL,
+    prior_sha256  TEXT NOT NULL,
+    prior_size    INTEGER NOT NULL,
+    PRIMARY KEY (beta_id, server, rel_path)
+);
+CREATE INDEX IF NOT EXISTS idx_beta_file_snapshots_beta ON beta_file_snapshots(beta_id);
+
 CREATE TABLE IF NOT EXISTS deployments (
     id                TEXT PRIMARY KEY,
     action            TEXT NOT NULL,                          -- 'drift' | 'promote' | 'rollback'