feat: add log collection and viewing system

Log Collectors (backend/internal/collectors/logs/):
- LogEntry model with level, source, message, fields
- Manager for coordinating multiple collectors
- JournalCollector: systemd journal via journalctl CLI
- FileCollector: tail log files with format parsing (plain, json, nginx)
- DockerCollector: docker container logs via docker CLI
- All collectors are pure Go (no CGO dependencies)

Database Storage:
- Add logs table with indexes for efficient querying
- StoreLogs: batch insert log entries
- QueryLogs: filter by agent, source, level, time, full-text search
- DeleteOldLogs: retention cleanup
- Implementations for both SQLite and PostgreSQL

Frontend Log Viewer:
- Log types and level color definitions
- Logs API client with streaming support
- /logs route with search, level filters, source filters
- Live streaming mode for real-time log tailing
- Paginated loading with load more

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/database/database.go

@@ -60,6 +60,11 @@ type Database interface {
 
 	// Retention
 	RunRetention(ctx context.Context) error
+
+	// Logs
+	StoreLogs(ctx context.Context, entries []LogEntry) error
+	QueryLogs(ctx context.Context, filter LogFilter) ([]LogEntry, int, error)
+	DeleteOldLogs(ctx context.Context, before time.Time) (int, error)
 }
 
 // MetricPoint represents a single metric data point.
@@ -182,6 +187,42 @@ type AlertFilter struct {
 	Offset       int
 }
 
+// LogLevel represents log severity.
+type LogLevel string
+
+const (
+	LogLevelDebug   LogLevel = "debug"
+	LogLevelInfo    LogLevel = "info"
+	LogLevelWarning LogLevel = "warning"
+	LogLevelError   LogLevel = "error"
+	LogLevelFatal   LogLevel = "fatal"
+)
+
+// LogEntry represents a stored log entry.
+type LogEntry struct {
+	ID         int64             `json:"id"`
+	AgentID    string            `json:"agentId"`
+	Timestamp  time.Time         `json:"timestamp"`
+	Source     string            `json:"source"`     // "journal", "file", "docker"
+	SourceName string            `json:"sourceName"` // Unit name, filename, container
+	Level      LogLevel          `json:"level"`
+	Message    string            `json:"message"`
+	Fields     map[string]string `json:"fields,omitempty"`
+}
+
+// LogFilter specifies criteria for querying logs.
+type LogFilter struct {
+	AgentID    string     // Filter by agent
+	Source     string     // Filter by source type (journal, file, docker)
+	SourceName string     // Filter by source name
+	Level      []LogLevel // Filter by levels
+	Query      string     // Full-text search query
+	From       time.Time
+	To         time.Time
+	Limit      int
+	Offset     int
+}
+
 // RetentionConfig defines data retention policies.
 type RetentionConfig struct {
 	// Raw metrics retention (default: 24 hours)
@@ -195,6 +236,9 @@ type RetentionConfig struct {
 
 	// Hourly aggregation retention (default: 1 year)
 	HourlyRetention time.Duration
+
+	// Log retention (default: 7 days)
+	LogRetention time.Duration
 }
 
 // DefaultRetentionConfig returns default retention settings.
@@ -204,5 +248,6 @@ func DefaultRetentionConfig() RetentionConfig {
 		OneMinuteRetention:  7 * 24 * time.Hour,
 		FiveMinuteRetention: 30 * 24 * time.Hour,
 		HourlyRetention:     365 * 24 * time.Hour,
+		LogRetention:        7 * 24 * time.Hour,
 	}
 }