feat(security): implement keystore for user authentication keys

* Add ConfigKeyStore for in-memory key management
* Introduce DatabaseKeyStore for PostgreSQL-backed key storage
* Create KeyStoreAuthenticator for API key validation
* Define SQL procedures for key management in PostgreSQL
* Document keystore functionality and usage in KEYSTORE.md

pkg/security/KEYSTORE.md

@@ -0,0 +1,153 @@
+# Keystore
+
+Per-user named auth keys with pluggable storage. Each user can hold multiple keys of different types — JWT secrets, header API keys, OAuth2 client credentials, or generic API keys. Keys are identified by a human-readable name ("CI deploy", "mobile app") and can carry scopes and arbitrary metadata.
+
+## Key types
+
+| Constant | Value | Use case |
+|---|---|---|
+| `KeyTypeJWTSecret` | `jwt_secret` | Per-user JWT signing secret |
+| `KeyTypeHeaderAPI` | `header_api` | Static API key sent in a request header |
+| `KeyTypeOAuth2` | `oauth2` | OAuth2 client credentials |
+| `KeyTypeGenericAPI` | `api` | General-purpose application key |
+
+## Storage backends
+
+### ConfigKeyStore
+
+In-memory store seeded from a static list. Suitable for a small, fixed set of service-account keys loaded from a config file. Keys created at runtime via `CreateKey` are held in memory and lost on restart.
+
+```go
+// Pre-load keys from config (KeyHash = SHA-256 hex of the raw key)
+store := security.NewConfigKeyStore([]security.UserKey{
+    {
+        UserID:  1,
+        KeyType: security.KeyTypeGenericAPI,
+        KeyHash: "e3b0c44298fc1c149afb...", // sha256(rawKey)
+        Name:    "CI deploy",
+        Scopes:  []string{"deploy"},
+        IsActive: true,
+    },
+})
+```
+
+### DatabaseKeyStore
+
+Backed by PostgreSQL stored procedures. Supports optional caching (default 2-minute TTL). Apply `keystore_schema.sql` before use.
+
+```go
+db, _ := sql.Open("postgres", dsn)
+
+store := security.NewDatabaseKeyStore(db)
+
+// With options
+store = security.NewDatabaseKeyStore(db, security.DatabaseKeyStoreOptions{
+    CacheTTL: 5 * time.Minute,
+    SQLNames: &security.KeyStoreSQLNames{
+        ValidateKey: "myapp_keystore_validate", // override one procedure name
+    },
+})
+```
+
+## Managing keys
+
+```go
+ctx := context.Background()
+
+// Create — raw key returned once; store it securely
+resp, err := store.CreateKey(ctx, security.CreateKeyRequest{
+    UserID:  42,
+    KeyType: security.KeyTypeGenericAPI,
+    Name:    "mobile app",
+    Scopes:  []string{"read", "write"},
+})
+fmt.Println(resp.RawKey) // only shown here; hashed internally
+
+// List
+keys, err := store.GetUserKeys(ctx, 42, "") // "" = all types
+keys, err  = store.GetUserKeys(ctx, 42, security.KeyTypeGenericAPI)
+
+// Revoke
+err = store.DeleteKey(ctx, 42, resp.Key.ID)
+
+// Validate (used by authenticators internally)
+key, err := store.ValidateKey(ctx, rawKey, "")
+```
+
+## HTTP authentication
+
+`KeyStoreAuthenticator` wraps any `KeyStore` and implements the `Authenticator` interface. It is drop-in compatible with `DatabaseAuthenticator` and works in `CompositeSecurityProvider`.
+
+Keys are extracted from the request in this order:
+
+1. `Authorization: Bearer <key>`
+2. `Authorization: ApiKey <key>`
+3. `X-API-Key: <key>`
+
+```go
+auth := security.NewKeyStoreAuthenticator(store, "") // "" = accept any key type
+// Restrict to a specific type:
+auth = security.NewKeyStoreAuthenticator(store, security.KeyTypeGenericAPI)
+```
+
+Plug it into a handler:
+
+```go
+handler := resolvespec.NewHandler(db, registry,
+    resolvespec.WithAuthenticator(auth),
+)
+```
+
+`Login` and `Logout` return an error — key lifecycle is managed through `KeyStore` directly.
+
+On successful validation the request context receives a `UserContext` where:
+
+- `UserID` — from the key
+- `Roles` — the key's `Scopes`
+- `Claims["key_type"]` — key type string
+- `Claims["key_name"]` — key name
+
+## Database setup
+
+Apply `keystore_schema.sql` to your PostgreSQL database. It requires the `users` table from the main `database_schema.sql`.
+
+```sql
+\i pkg/security/keystore_schema.sql
+```
+
+This creates:
+
+- `user_keys` table with indexes on `user_id`, `key_hash`, and `key_type`
+- `resolvespec_keystore_get_user_keys(p_user_id, p_key_type)`
+- `resolvespec_keystore_create_key(p_request jsonb)`
+- `resolvespec_keystore_delete_key(p_user_id, p_key_id)`
+- `resolvespec_keystore_validate_key(p_key_hash, p_key_type)`
+
+### Custom procedure names
+
+```go
+store := security.NewDatabaseKeyStore(db, security.DatabaseKeyStoreOptions{
+    SQLNames: &security.KeyStoreSQLNames{
+        GetUserKeys: "myschema_get_keys",
+        CreateKey:   "myschema_create_key",
+        DeleteKey:   "myschema_delete_key",
+        ValidateKey: "myschema_validate_key",
+    },
+})
+
+// Validate names at startup
+names := &security.KeyStoreSQLNames{
+    GetUserKeys: "myschema_get_keys",
+    // ...
+}
+if err := security.ValidateKeyStoreSQLNames(names); err != nil {
+    log.Fatal(err)
+}
+```
+
+## Security notes
+
+- Raw keys are never stored. Only the SHA-256 hex digest is persisted.
+- The raw key is generated with `crypto/rand` (32 bytes, base64url-encoded) and returned exactly once in `CreateKeyResponse.RawKey`.
+- Hash comparisons in `ConfigKeyStore` use `crypto/subtle.ConstantTimeCompare` to prevent timing side-channels.
+- `DeleteKey` performs a soft delete (`is_active = false`). The `DatabaseKeyStore` invalidates the cache entry immediately, but due to the cache TTL a revoked key may authenticate for up to `CacheTTL` (default 2 minutes) in a distributed environment. Set `CacheTTL: 0` to disable caching if immediate revocation is required.