Add vault seal/unseal lifecycle

- New internal/vault package: thread-safe Vault struct with
  seal/unseal state, key material zeroing, and key derivation
- REST: POST /v1/vault/unseal, POST /v1/vault/seal,
  GET /v1/vault/status; health returns sealed status
- UI: /unseal page with passphrase form, redirect when sealed
- gRPC: sealedInterceptor rejects RPCs when sealed
- Middleware: RequireUnsealed blocks all routes except exempt
  paths; RequireAuth reads pubkey from vault at request time
- Startup: server starts sealed when passphrase unavailable
- All servers share single *vault.Vault by pointer
- CSRF manager derives key lazily from vault

Security: Key material is zeroed on seal. Sealed middleware
runs before auth. Handlers fail closed if vault becomes sealed
mid-request. Unseal endpoint is rate-limited (3/s burst 5).
No CSRF on unseal page (no session to protect; chicken-and-egg
with master key). Passphrase never logged.

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

internal/vault/derive.go

@@ -0,0 +1,67 @@
+package vault
+
+import (
+	"crypto/ed25519"
+	"errors"
+	"fmt"
+
+	"git.wntrmute.dev/kyle/mcias/internal/crypto"
+	"git.wntrmute.dev/kyle/mcias/internal/db"
+)
+
+// DeriveFromPassphrase derives the master encryption key from a passphrase
+// using the Argon2id KDF with a salt stored in the database.
+//
+// Security: The Argon2id parameters used by crypto.DeriveKey exceed OWASP 2023
+// minimums (time=3, memory=128MiB, threads=4). The salt is 32 random bytes
+// stored in the database on first run.
+func DeriveFromPassphrase(passphrase string, database *db.DB) ([]byte, error) {
+	salt, err := database.ReadMasterKeySalt()
+	if errors.Is(err, db.ErrNotFound) {
+		return nil, fmt.Errorf("no master key salt in database (first-run requires startup passphrase)")
+	}
+	if err != nil {
+		return nil, fmt.Errorf("read master key salt: %w", err)
+	}
+
+	key, err := crypto.DeriveKey(passphrase, salt)
+	if err != nil {
+		return nil, fmt.Errorf("derive master key: %w", err)
+	}
+	return key, nil
+}
+
+// DecryptSigningKey decrypts the Ed25519 signing key pair from the database
+// using the provided master key.
+//
+// Security: The private key is stored AES-256-GCM encrypted in the database.
+// A fresh random nonce is used for each encryption. The plaintext key only
+// exists in memory during the process lifetime.
+func DecryptSigningKey(database *db.DB, masterKey []byte) (ed25519.PrivateKey, ed25519.PublicKey, error) {
+	enc, nonce, err := database.ReadServerConfig()
+	if err != nil {
+		return nil, nil, fmt.Errorf("read server config: %w", err)
+	}
+	if enc == nil || nonce == nil {
+		return nil, nil, fmt.Errorf("no signing key in database (first-run requires startup passphrase)")
+	}
+
+	privPEM, err := crypto.OpenAESGCM(masterKey, nonce, enc)
+	if err != nil {
+		return nil, nil, fmt.Errorf("decrypt signing key: %w", err)
+	}
+
+	priv, err := crypto.ParsePrivateKeyPEM(privPEM)
+	if err != nil {
+		return nil, nil, fmt.Errorf("parse signing key PEM: %w", err)
+	}
+
+	// Security: ed25519.PrivateKey.Public() always returns ed25519.PublicKey,
+	// but we use the ok form to make the type assertion explicit and safe.
+	pub, ok := priv.Public().(ed25519.PublicKey)
+	if !ok {
+		return nil, nil, fmt.Errorf("signing key has unexpected public key type")
+	}
+
+	return priv, pub, nil
+}