feat(auth): argon2id hashing + AuthUser/Authorized<Cap> session extractors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [workspace]
 resolver = "3"
-members = ["crates/domain", "crates/db", "crates/api", "crates/server", "crates/search"]
+members = ["crates/domain", "crates/db", "crates/api", "crates/server", "crates/search", "crates/auth"]
 
 [workspace.package]
 edition = "2024"
@@ -24,3 +24,7 @@ tower = { version = "0.5", features = ["util"] }
 http-body-util = "0.1"
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls"] }
 meilisearch-sdk = "0.33"
+argon2 = "0.5"
+tower-sessions = "0.14"
+tower-sessions-sqlx-store = { version = "0.15", features = ["postgres"] }
+rpassword = "7"