build: use bun for installs and pin better-auth core deps

Railpack builds the Dokploy app from the app's build path, which is
apps/web — a dir with no bun.lock (it lives at the monorepo root), so
Railpack fell back to npm. npm then resolved @better-auth/core's ^1.6.22
peer to 1.7.0 while the pinned plugins peer better-call@1.3.7, causing
ERESOLVE and failing the build.

- Declare packageManager: bun@1.3.14 at the root (Railpack's
  highest-priority signal; combined with removing the Dokploy build path,
  installs run 'bun install --frozen-lockfile' at the workspace root).
- Add npm overrides (@better-auth/core 1.6.22, better-call 1.3.7) in
  root + apps/web as a safety net so any npm-based build resolves too.
- Verified: isolated npm install resolves (698 pkgs), bun install is a
  no-op (1020 pkgs, versions unchanged), full next build passes.

bun.lock

@@ -106,8 +106,10 @@
     "unrs-resolver",
   ],
   "overrides": {
+    "@better-auth/core": "1.6.22",
     "@types/react": "19.2.14",
     "@types/react-dom": "19.2.3",
+    "better-call": "1.3.7",
   },
   "packages": {
     "@alloc/quick-lru": ["@alloc/quick-lru@5.2.0", "", {}, "sha512-UrcABB+4bUrFABwbluTIBErXwvbsU/V7TZWfmbgJfbkwiBuziS9gxdODUyuiecfdGQ85jglMW6juS3+z5TsKLw=="],