feat(wfe-postgres): add PostgreSQL persistence provider

PostgresPersistenceProvider using sqlx with wfc schema. JSONB for
complex fields, TIMESTAMPTZ for dates. Transactions for atomicity.
Proper error propagation on deserialization (no unwrap_or_default).
Status conversion returns Result for unknown values.

wfe-postgres/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "wfe-postgres"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "PostgreSQL persistence provider for WFE"
+
+[dependencies]
+wfe-core = { workspace = true }
+sqlx = { workspace = true }
+tokio = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+async-trait = { workspace = true }
+uuid = { workspace = true }
+chrono = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }
+
+[dev-dependencies]
+wfe-core = { workspace = true, features = ["test-support"] }
+pretty_assertions = { workspace = true }
+rstest = { workspace = true }
+tokio = { workspace = true, features = ["test-util"] }