feat(cluster): persist learnt locators behind a "Chase new grids" option

The callsign->grid map died with the process. Every restart began with an empty
Locator column that took an hour of listening to refill, and everything learnt
the day before was thrown away.

internal/gridcache is its own SQLite file in the data directory, not a table in
the settings database: that one sits wherever the operator chose to put it,
often a synchronised folder, and a store that rewrites itself every minute has
no business there. Deleting the file costs a few days of listening and nothing
else.

Callsign is the primary key and the newest report wins — operators move, go
portable, go on expedition, and a stale locator is worse than none for grid
chasing because it reads as a square already worked. Entries not seen in two
years are pruned at open: that is the one way this cache can be actively wrong
rather than merely empty, and it is what bounds a store that would otherwise
only grow.

Only CHANGES are queued. The feeds repeat themselves, so writing every report
would put the whole stream in the batch instead of the news in it. Batches
flush on a timer and on shutdown — a restart is exactly when the cache is worth
the most.

Rotation is switched off while the store is attached: the cap would discard
callsigns the database still holds and the lookup would then miss what we know.
Age in the store is the bound instead.

decodegrids_test.go

@@ -2,8 +2,11 @@ package main
 
 import (
 	"fmt"
+	"path/filepath"
 	"sync"
 	"testing"
+
+	"hamlog/internal/gridcache"
 )
 
 // The grid cache used to drop EVERYTHING at its ceiling. That was survivable
@@ -95,3 +98,63 @@ func TestDecodeGridConcurrentAccess(t *testing.T) {
 	}()
 	wg.Wait()
 }
+
+// Only a CHANGE may reach the write batch.
+//
+// The feeds repeat themselves — the same station is reported by dozens of
+// receivers a minute — so queueing every report would put the whole stream in
+// the batch instead of the news in it, and turn a cache into a write amplifier.
+func TestOnlyChangesAreQueued(t *testing.T) {
+	st, err := gridcache.Open(filepath.Join(t.TempDir(), "grids.db"), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer st.Close()
+	a := &App{gridStore: st}
+
+	a.rememberDecodeGrid("F4BPO", "JN36")
+	if n := st.Pending(); n != 1 {
+		t.Fatalf("a new locator queued %d writes, want 1", n)
+	}
+	if err := st.Flush(); err != nil {
+		t.Fatal(err)
+	}
+
+	// The same report, a hundred times over, is not news.
+	for i := 0; i < 100; i++ {
+		a.rememberDecodeGrid("F4BPO", "JN36")
+	}
+	if n := st.Pending(); n != 0 {
+		t.Errorf("unchanged reports queued %d writes — the batch would carry the whole feed", n)
+	}
+
+	// A station that moved is.
+	a.rememberDecodeGrid("F4BPO", "KP30")
+	if n := st.Pending(); n != 1 {
+		t.Errorf("a changed locator queued %d writes, want 1", n)
+	}
+	if got := a.lookupDecodeGrid("F4BPO"); got != "KP30" {
+		t.Errorf("the map kept the old locator: %q", got)
+	}
+}
+
+// Rotation must be OFF while a store is attached: it would discard callsigns the
+// database still holds, and the lookup would then miss something we know.
+func TestNoRotationWhilePersisting(t *testing.T) {
+	st, err := gridcache.Open(filepath.Join(t.TempDir(), "grids.db"), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer st.Close()
+	a := &App{gridStore: st}
+
+	for i := 0; i < decodeGridsCap+10; i++ {
+		a.rememberDecodeGrid(fmt.Sprintf("CALL%06d", i), "JN36")
+	}
+	if a.decodeGridsOld != nil {
+		t.Error("rotated while a store was attached — locators the database holds would go missing")
+	}
+	if got := a.lookupDecodeGrid("CALL000000"); got != "JN36" {
+		t.Errorf("the first entry was dropped: %q", got)
+	}
+}