Add exchange rate service with caching and fallback API

ExchangeRateService fetches rates from fawazahmed0/exchange-api with
Frankfurter API as fallback. Rates are cached in SQLite for 24 hours.
Includes response parsers, 30 supported currencies, and 8 unit tests
covering parsing, caching, and same-currency identity.

outlay-core/src/db.rs

@@ -387,6 +387,39 @@ impl Database {
         rows.collect()
     }
 
+    // -- Exchange Rates --
+
+    pub fn get_cached_rate(&self, base: &str, target: &str) -> SqlResult<Option<ExchangeRate>> {
+        match self.conn.query_row(
+            "SELECT base, target, rate, fetched_at FROM exchange_rates WHERE base = ?1 AND target = ?2",
+            params![base.to_lowercase(), target.to_lowercase()],
+            |row| {
+                Ok(ExchangeRate {
+                    base: row.get(0)?,
+                    target: row.get(1)?,
+                    rate: row.get(2)?,
+                    fetched_at: row.get(3)?,
+                })
+            },
+        ) {
+            Ok(rate) => Ok(Some(rate)),
+            Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+            Err(e) => Err(e),
+        }
+    }
+
+    pub fn cache_rates(&self, base: &str, rates: &std::collections::HashMap<String, f64>) -> SqlResult<()> {
+        let now = chrono::Utc::now().to_rfc3339();
+        let base_lower = base.to_lowercase();
+        let mut stmt = self.conn.prepare(
+            "INSERT OR REPLACE INTO exchange_rates (base, target, rate, fetched_at) VALUES (?1, ?2, ?3, ?4)"
+        )?;
+        for (target, rate) in rates {
+            stmt.execute(params![base_lower, target.to_lowercase(), rate, now])?;
+        }
+        Ok(())
+    }
+
     // -- Settings --
 
     pub fn get_setting(&self, key: &str) -> SqlResult<Option<String>> {