release(checkpoint-postgres): pin to checkpoint 4.1.0a1 (#7648)

## Summary
- Bumps `langgraph-checkpoint-postgres`'s pin on `langgraph-checkpoint`
from `>=4.0.3,<5.0.0` to `>=4.1.0a1,<5.0.0` so the postgres alpha
(3.1.0a1) requires the matching checkpoint alpha released in #7647.
- Mirrors the same pin update applied to `langgraph` (1.2.0a1) on
`wfh/releases/timers`.

## Why
The three alphas (checkpoint 4.1.0a1, checkpoint-postgres 3.1.0a1,
langgraph 1.2.0a1) are meant to be tested as a coherent set. `langgraph`
already pins `>=4.1.0a1`; checkpoint-postgres was missed in that release
and is still letting resolvers fall back to 4.0.x.

## Test plan
- [ ] CI green
- [ ] `uv lock` resolves cleanly across libs (verified locally via `make
lock` — no lock file changes since editable paths already resolved to
4.1.0a1)

libs/checkpoint-postgres/pyproject.toml

@@ -12,7 +12,7 @@ readme = "README.md"
 license = "MIT"
 license-files = ['LICENSE']
 dependencies = [
-  "langgraph-checkpoint>=4.0.3,<5.0.0",
+  "langgraph-checkpoint>=4.1.0a1,<5.0.0",
   "orjson>=3.11.5",
   "psycopg>=3.2.0",
   "psycopg-pool>=3.2.0",