Add initial backend implementation

- Go module with Gin, GORM, JWT, excelize dependencies
- Configuration loading from YAML with all settings
- GORM models for users, categories, components, configurations, alerts
- Repository layer for all entities
- Services: auth (JWT), pricing (median/average/weighted), components,
  quotes, configurations, export (CSV/XLSX), alerts
- Middleware: JWT auth, role-based access, CORS
- HTTP handlers for all API endpoints
- Main server with dependency injection and graceful shutdown

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

config.example.yaml

@@ -0,0 +1,58 @@
+# QuoteForge Configuration
+# Copy this file to config.yaml and update values
+
+server:
+  host: "0.0.0.0"
+  port: 8080
+  mode: "release"  # debug | release
+  read_timeout: "30s"
+  write_timeout: "30s"
+
+database:
+  host: "localhost"
+  port: 3306
+  name: "RFQ_LOG"
+  user: "quoteforge"
+  password: "CHANGE_ME"
+  max_open_conns: 25
+  max_idle_conns: 5
+  conn_max_lifetime: "5m"
+
+auth:
+  jwt_secret: "CHANGE_ME_MIN_32_CHARACTERS_LONG"
+  token_expiry: "24h"
+  refresh_expiry: "168h"  # 7 days
+
+pricing:
+  default_method: "weighted_median"  # median | average | weighted_median
+  default_period_days: 90
+  freshness_green_days: 30
+  freshness_yellow_days: 60
+  freshness_red_days: 90
+  min_quotes_for_median: 3
+  popularity_decay_days: 180
+
+export:
+  temp_dir: "/tmp/quoteforge-exports"
+  max_file_age: "1h"
+  company_name: "Your Company Name"
+
+alerts:
+  enabled: true
+  check_interval: "1h"
+  high_demand_threshold: 5      # КП за 30 дней
+  trending_threshold_percent: 50 # % роста для алерта
+
+notifications:
+  email_enabled: false
+  smtp_host: "smtp.example.com"
+  smtp_port: 587
+  smtp_user: ""
+  smtp_password: ""
+  from_address: "quoteforge@example.com"
+
+logging:
+  level: "info"  # debug | info | warn | error
+  format: "json" # json | text
+  output: "stdout" # stdout | file
+  file_path: "/var/log/quoteforge/app.log"