feat: add SQLite storage for login attempts and sessions

Adds persistent storage using modernc.org/sqlite (pure Go). Login
attempts are deduplicated by (username, password, ip) with counts.
Sessions and session logs are tracked with UUID IDs. Includes embedded
SQL migrations, configurable retention with background pruning, and
an in-memory store for tests.

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

internal/config/config.go

@@ -9,12 +9,22 @@ import (
 )
 
 type Config struct {
-	SSH      SSHConfig  `toml:"ssh"`
-	Auth     AuthConfig `toml:"auth"`
+	SSH      SSHConfig      `toml:"ssh"`
+	Auth     AuthConfig     `toml:"auth"`
+	Storage  StorageConfig  `toml:"storage"`
 	LogLevel  string `toml:"log_level"`
 	LogFormat string `toml:"log_format"` // "text" (default) or "json"
 }
 
+type StorageConfig struct {
+	DBPath            string `toml:"db_path"`
+	RetentionDays     int    `toml:"retention_days"`
+	RetentionInterval string `toml:"retention_interval"`
+
+	// Parsed duration, not from TOML directly.
+	RetentionIntervalDuration time.Duration `toml:"-"`
+}
+
 type SSHConfig struct {
 	ListenAddr     string `toml:"listen_addr"`
 	HostKeyPath    string `toml:"host_key_path"`
@@ -77,6 +87,15 @@ func applyDefaults(cfg *Config) {
 	if cfg.LogFormat == "" {
 		cfg.LogFormat = "text"
 	}
+	if cfg.Storage.DBPath == "" {
+		cfg.Storage.DBPath = "oubliette.db"
+	}
+	if cfg.Storage.RetentionDays == 0 {
+		cfg.Storage.RetentionDays = 90
+	}
+	if cfg.Storage.RetentionInterval == "" {
+		cfg.Storage.RetentionInterval = "1h"
+	}
 }
 
 func validate(cfg *Config) error {
@@ -93,6 +112,19 @@ func validate(cfg *Config) error {
 		return fmt.Errorf("accept_after must be at least 1, got %d", cfg.Auth.AcceptAfter)
 	}
 
+	ri, err := time.ParseDuration(cfg.Storage.RetentionInterval)
+	if err != nil {
+		return fmt.Errorf("invalid retention_interval %q: %w", cfg.Storage.RetentionInterval, err)
+	}
+	if ri <= 0 {
+		return fmt.Errorf("retention_interval must be positive, got %s", ri)
+	}
+	cfg.Storage.RetentionIntervalDuration = ri
+
+	if cfg.Storage.RetentionDays < 1 {
+		return fmt.Errorf("retention_days must be at least 1, got %d", cfg.Storage.RetentionDays)
+	}
+
 	for i, cred := range cfg.Auth.StaticCredentials {
 		if cred.Username == "" {
 			return fmt.Errorf("static_credentials[%d]: username must not be empty", i)