feat(storage): durable Postgres store + shipped migrations (FOR UPDATE SKIP LOCKED)

[williamdewitt]

Part of #8 (M2 epic). The scale-up provider. Ships EF migrations → risk:critical (migration-safety review on the PR).

Acceptance criteria

• IMessageStore + IEndpointStore on Postgres via EF Core, with shipped, reviewed migrations for the messages/endpoints schema (no ensure-created in production).
• Claim path uses SELECT ... FOR UPDATE SKIP LOCKED so N dispatchers claim disjoint batches with no contention and no double-send.
• Same IMessageStore contract + semantics as the SQLite/in-memory tiers (the M1 benchmark on ClaimDueAsync is the reference for the SQL push-down).
• One-line opt-in (AddCaliberWebhooks(o => o.UsePostgres(...)) / UseEntityFramework<AppDbContext>()), guarantee tier explicit.

Definition of done

• Builds net8.0 + net10.0, zero warnings; migration SQL reviewed for safety; integration tests (see the Testcontainers issue) prove no-double-send; docs reconciled.

Design: docs/design/caliber-webhooks-core-design.md §Work-claiming data model • concepts/storage-and-work-claiming.md • roadmap M2. Depends on the SQLite store shaping the contract.