fix: standardize TypeScript to ^5.9.3 across all packages

Closes #600

- Root, backend, and sdk were pinned to ^6.0.3 (a pre-release/nightly
  build not published as a stable npm release), causing toolchain
  instability across packages that share types.
- Align all five packages to ^5.9.3 (current stable).
- Add docs/TYPESCRIPT_POLICY.md documenting the canonical version,
  rationale, and upgrade procedure.

Author: AnnieIj

backend/package.json

@@ -95,6 +95,6 @@
     "supertest": "^7.2.2",
     "ts-jest": "^29.4.9",
     "ts-node-dev": "^2.0.0",
-    "typescript": "^6.0.3"
+    "typescript": "^5.9.3"
   }
 }

docs/TYPESCRIPT_POLICY.md

@@ -0,0 +1,30 @@
+# TypeScript Version Policy
+
+## Canonical Version
+
+All packages in this monorepo pin to **TypeScript `^5.9.3`**.
+
+| Package   | Version   |
+|-----------|-----------|
+| root      | `^5.9.3`  |
+| backend   | `^5.9.3`  |
+| client    | `^5.9.3`  |
+| sdk       | `^5.9.3`  |
+| shared    | `^5.9.3`  |
+
+## Rationale
+
+- TypeScript 5.x is the current stable major version.
+- `^6.0.3` referenced a pre-release / nightly build that does not exist on the public npm registry as a stable release, causing toolchain instability (issue #600).
+- A single version eliminates version-skew bugs between packages that share types via `@syncro/shared`.
+
+## Upgrade Policy
+
+1. Upgrades are evaluated when a new TypeScript **stable** minor or major is released.
+2. All packages are updated together in a single PR.
+3. The PR must include a passing typecheck across every package (`npm run typecheck --workspaces`).
+4. Deviations (e.g. a package temporarily pinned to an older patch) must be documented here with a justification and a target remediation date.
+
+## Current Deviations
+
+None.