Big change

[dbrowne]

• New crypto-loaders crate: Extracted cryptocurrency-specific loaders into a standalone crate with multi-provider support (CoinGecko, CoinMarketCap, CoinPaprika, CoinCap, SosoValue). Includes symbol discovery, metadata, details, social metrics, and cross-provider mapping services.
• Unified cache implementation: Added a shared cache.rs module in av-loaders consolidating response caching logic across all loaders.
• av-core type refactor: Split monolithic market.rs into focused modules (classifications.rs, exchange.rs, security_type.rs). Converted all from_str inherent methods to proper FromStr trait implementations.
• av-loaders cleanup: Refactored crypto loaders to delegate to the new crypto-loaders crate, added metadata provider abstraction layer, and removed redundant .map_err(Into::into) conversions.
• Dependency updates: Upgraded Diesel 2.2 to 2.3, diesel-async 0.6 to 0.8, diesel_migrations 2.2 to 2.3 — resolves ambiguous glob import warnings for max/sum.
• Suppressed async_fn_in_trait warning on CacheRepositoryExt (generic methods are non-object-safe, so Send bound is irrelevant).
• Updated README to reflect current codebase structure, CLI commands, and dependencies.

Test plan

• cargo check passes with no warnings
• cargo test -p av-core — all 88 unit tests pass
• cargo clippy — no new warnings introduced
• Verify crypto loader CLI commands function against live APIs