Extract SlashingDatabase trait to enable mock testing

[Copilot]

Extracts a SlashingDatabase trait from the concrete SQLite implementation to enable testing with mocks or instrumentation wrappers.

Changes

• Created SlashingDatabase trait containing all public methods (validation, registration, import/export, pruning)
• Renamed struct SlashingDatabase → SqliteSlashingDatabase
• Made LighthouseValidatorStore generic over the trait with Send + Sync + 'static bounds
• Updated all call sites to use SqliteSlashingDatabase directly or accept generic S: SlashingDatabase

Example

// Before
pub struct LighthouseValidatorStore<T, E> {
    slashing_protection: SlashingDatabase,
    // ...
}

// After - generic over any SlashingDatabase implementation
pub struct LighthouseValidatorStore<T, E, S> {
    slashing_protection: S,
    // ...
}
impl<T, E, S: SlashingDatabase + Send + Sync> LighthouseValidatorStore<T, E, S> {
    // ...
}

// Production code uses concrete type
let db = SqliteSlashingDatabase::open_or_create(path)?;
let store = LighthouseValidatorStore::new(validators, db, ...);

// Tests can inject mocks
let mock_db = MockSlashingDatabase::new();
let store = LighthouseValidatorStore::new(validators, mock_db, ...);

Both trait and concrete type are re-exported from slashing_protection crate for backward compatibility.

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