feat(manager-core): admin auth gate (Phase 3a)

Closes the regression risk of the admin API and dashboard being open
to anyone reaching the bound port. Required foundation before v1.1
data-plane services land.

Per-user accounts (admin_users), Argon2id passwords, env-var bootstrap
of the first admin that becomes inert once any admin exists, opaque
32-byte session token doubling as bearer credential, 24h sliding TTL
configurable via PICLOUD_SESSION_TTL_HOURS. is_active column lets
admins be deactivated without losing audit history; last-active-admin
guard on DELETE and on PATCH that flips is_active to false (sessions
also wiped on deactivation).

require_admin middleware fronts every /api/v1/admin/* route. The data
plane (/api/v1/execute/{id}), /healthz, /version, and user routes
stay open. picloud admin reset-password <username> subcommand handles
recovery without going through HTTP.

Dashboard gains /admin/login and /admin/admins surfaces, a top-bar
user menu, and a token store with a localStorage echo so refreshes
don't sign you out. Cookie-based auth works in parallel for non-SPA
clients.

Forward compatibility: future RBAC tables (admin_roles,
admin_user_roles) join on admin_users.id; the auth middleware is the
seam where role checks slot in. Email, 2FA, passkeys, and personal
API tokens are all additive without touching admin_users.

Blueprint §11.4 updated to reflect what actually shipped.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/manager-core/src/auth.rs

@@ -0,0 +1,132 @@
+//! Pure auth helpers: password hashing, session-token generation, and
+//! token-to-hash conversion. No DB, no HTTP — repos and middleware live
+//! in their own modules. Keeping this surface pure also keeps the unit
+//! tests fast (no Postgres needed).
+//!
+//! Hash algorithm is Argon2id with the OWASP default parameters
+//! (`Argon2::default()`). Tokens are 32 cryptographically random bytes
+//! base64-url-encoded for the wire; their SHA-256 (hex) is what hits the
+//! sessions table.
+
+use argon2::password_hash::rand_core::OsRng as ArgonRng;
+use argon2::password_hash::{PasswordHash, PasswordHasher, PasswordVerifier, SaltString};
+use argon2::Argon2;
+use base64::engine::general_purpose::URL_SAFE_NO_PAD;
+use base64::Engine as _;
+use rand::rngs::OsRng;
+use rand::RngCore;
+use sha2::{Digest, Sha256};
+
+/// Returned when the supplied password hash string isn't a valid PHC
+/// Argon2id encoding. Only surfaces at bootstrap time when the operator
+/// passes `PICLOUD_ADMIN_PASSWORD_HASH`.
+#[derive(Debug, thiserror::Error)]
+#[error("invalid Argon2id PHC hash")]
+pub struct InvalidPasswordHash;
+
+/// Hash a raw password into an Argon2id PHC-formatted string suitable
+/// for `admin_users.password_hash`. The output already encodes the salt
+/// and parameters; nothing else needs to be persisted alongside it.
+pub fn hash_password(raw: &str) -> Result<String, argon2::password_hash::Error> {
+    let salt = SaltString::generate(&mut ArgonRng);
+    let hash = Argon2::default().hash_password(raw.as_bytes(), &salt)?;
+    Ok(hash.to_string())
+}
+
+/// Constant-ish-time verify of a raw password against a PHC hash.
+/// Returns `false` for any error (including malformed stored hash) —
+/// callers should treat that case identically to "wrong password" so
+/// nothing leaks about why auth failed.
+#[must_use]
+pub fn verify_password(stored_hash: &str, raw: &str) -> bool {
+    let Ok(parsed) = PasswordHash::new(stored_hash) else {
+        return false;
+    };
+    Argon2::default()
+        .verify_password(raw.as_bytes(), &parsed)
+        .is_ok()
+}
+
+/// Validate that a string parses as a PHC Argon2id hash — used at
+/// bootstrap to fail fast on malformed `PICLOUD_ADMIN_PASSWORD_HASH`
+/// rather than write garbage into the DB and discover it at first login.
+pub fn validate_password_hash(stored_hash: &str) -> Result<(), InvalidPasswordHash> {
+    PasswordHash::new(stored_hash).map_err(|_| InvalidPasswordHash)?;
+    Ok(())
+}
+
+/// Newly minted session token: `raw` goes to the client (cookie + JSON
+/// response), `hash` is what gets stored. Raw is unrecoverable from hash
+/// even if the DB leaks.
+pub struct GeneratedToken {
+    pub raw: String,
+    pub hash: String,
+}
+
+/// Generate a fresh session token (32 random bytes base64-url-encoded).
+/// Always succeeds — `OsRng::fill_bytes` panics on entropy failure
+/// instead of returning, but that's a non-recoverable system condition.
+#[must_use]
+pub fn generate_session_token() -> GeneratedToken {
+    let mut bytes = [0u8; 32];
+    OsRng.fill_bytes(&mut bytes);
+    let raw = URL_SAFE_NO_PAD.encode(bytes);
+    let hash = hash_token(&raw);
+    GeneratedToken { raw, hash }
+}
+
+/// SHA-256(raw) as lower-case hex. Stable lookup key for
+/// `admin_sessions.token_hash`.
+#[must_use]
+pub fn hash_token(raw: &str) -> String {
+    let digest = Sha256::digest(raw.as_bytes());
+    hex(&digest)
+}
+
+fn hex(bytes: &[u8]) -> String {
+    const HEX: &[u8; 16] = b"0123456789abcdef";
+    let mut out = String::with_capacity(bytes.len() * 2);
+    for &b in bytes {
+        out.push(HEX[(b >> 4) as usize] as char);
+        out.push(HEX[(b & 0x0f) as usize] as char);
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn hash_verify_roundtrip() {
+        let h = hash_password("correct horse battery staple").unwrap();
+        assert!(verify_password(&h, "correct horse battery staple"));
+        assert!(!verify_password(&h, "wrong"));
+    }
+
+    #[test]
+    fn verify_returns_false_on_malformed_hash() {
+        assert!(!verify_password("not-a-phc-string", "anything"));
+    }
+
+    #[test]
+    fn validate_password_hash_accepts_phc() {
+        let h = hash_password("pw").unwrap();
+        assert!(validate_password_hash(&h).is_ok());
+    }
+
+    #[test]
+    fn validate_password_hash_rejects_garbage() {
+        assert!(validate_password_hash("not a hash").is_err());
+    }
+
+    #[test]
+    fn generate_token_unique_and_hash_stable() {
+        let a = generate_session_token();
+        let b = generate_session_token();
+        assert_ne!(a.raw, b.raw, "tokens must be unique");
+        assert_ne!(a.hash, b.hash, "hashes must differ");
+        assert_eq!(a.hash, hash_token(&a.raw), "hash must be reproducible");
+        assert_eq!(a.hash.len(), 64, "sha256-hex is 64 chars");
+    }
+}