feat: implement config hot-reload and fix corrupted backend files

[malik203]

feat: implement config hot-reload and fix corrupted backend files

Summary

Implements runtime configuration hot-reload for the backend and repairs all files that had
multiple versions concatenated together.

Changes

Config hot-reload (backend/src/config/)

• AppConfig in config/mod.rs — hot-reloadable struct with ServerConfig, DatabaseConfig,
  RedisConfig, log_level, max_connections, request_timeout_secs, maintenance_mode
• ConfigManager — wraps AppConfig in ArcSwap for lock-free reads; supports full reload from
  config.json and partial JSON-patch updates via update_from_patch
• ConfigWatcher — subscribes to config:reload Redis pub/sub channel, fetches updated config
  from config:current key, and atomically swaps the in-memory value; notifies all ConfigHandle
  holders via a tokio::sync::watch channel
• ConfigHandle — cheap-to-clone reader with get() and wait_for_change() support
• ReloadError — unified error type covering Redis, I/O, deserialisation, and validation
  failures
• handle_reload / handle_get_config — Axum handlers for POST /api/config/reload and GET
  /api/config

File corruption fixes

• Cargo.toml — removed duplicate [dependencies] / [dev-dependencies] blocks and conflicting
  dep versions
• src/lib.rs, src/error.rs, src/jobs.rs, src/telemetry.rs — removed concatenated duplicate
  implementations
• src/services/mod.rs, src/api/mod.rs, src/api/handlers/mod.rs, src/api/middleware/mod.rs —
  deduplicated module declarations
• src/api/handlers/profiling.rs, src/api/handlers/dashboard.rs — single clean implementation
  each

Testing

Unit tests in config/reload.rs cover:

• ConfigManager: load, reload (missing file error path), patch (top-level and nested fields,
  field preservation)
• ConfigWatcher: reload updates config, unchanged config skips notification, changed config
  notifies handles, multiple handles see same update, wait_for_change resolves after reload
• reload_from_redis: connection error leaves config unchanged
• ReloadError: display formatting for all variants
• AppConfig: default values, serialisation roundtrip
• closes [Backend] 10.8 Build Configuration Hot-Reload #184

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