Added database.change_batch_retention, deleting old change batches

Each save that records a batch also deletes, in the same
transaction, the batches older than database.change_batch_retention
(30 days by default; 0 or less keeps them for ever), with their
records of opportunities. A client still holding a deleted batch
gets 410 Gone from ?changed_after= and reloads the whole list, as
for any unknown batch.

internal/config/README.md

@@ -58,6 +58,7 @@ priority over `--quiet`.
 | Key | Type | Default | Notes |
 |---|---|---|---|
 | `database.file` | string | | Path to the SQLite database file. Empty: commands that need the database fail |
+| `database.change_batch_retention` | duration | `720h` | How long a batch of changed opportunities is kept for clients to catch up from (`?changed_after=`); older ones are deleted as new ones are recorded. A client whose batch has gone reloads the whole list. 0 or less: kept for ever |
 
 Database is currently restricted to SQLite v3. At this point, no other engine is
 supported.
@@ -219,6 +220,7 @@ logging:
     file: ""
 database:
     file: ./jobsearch.db
+    change_batch_retention: 720h
 sources:
     find_a_tender:
         enabled: true

internal/config/conf_database.go

@@ -1,15 +1,21 @@
 package config
 
-import "reflect"
+import (
+	"reflect"
+	"time"
+)
 
 // JSCKDatabase is the database section's configuration key.
 const JSCKDatabase string = "database"
 
 // JSCDatabase configures the SQLite database. File is the path to
 // the database file; empty means none, and commands that need the database
-// fail with a message saying so.
+// fail with a message saying so. ChangeBatchRetention is how long a batch of
+// changed opportunities is kept for clients to catch up from (see
+// store.WithChangeBatchRetention); 0 or less means they are kept for ever.
 type JSCDatabase struct {
-	File string `cfg:"file"`
+	File                 string        `cfg:"file"`
+	ChangeBatchRetention time.Duration `cfg:"change_batch_retention" default:"720h"`
 }
 
 // ---------- Section methods (see Section) ----------

internal/config/conf_sections_test.go

@@ -595,18 +595,24 @@ func TestLoadFromViperInvalidValues(t *testing.T) {
 
 func TestDatabaseSection(t *testing.T) {
 	c := JSCDatabase{
-		File: "./jobsearch.db",
+		File:                 "./jobsearch.db",
+		ChangeBatchRetention: 48 * time.Hour,
 	}
 
-	if got, want := c.Keys(), []string{"file"}; !reflect.DeepEqual(got, want) {
-		t.Errorf("Keys = %v; want %v", got, want)
+	keys := []string{"file", "change_batch_retention"}
+	if got := c.Keys(); !reflect.DeepEqual(got, keys) {
+		t.Errorf("Keys = %v; want %v", got, keys)
 	}
 	if got, want := c.Map(), map[string]any{
-		"file": "./jobsearch.db",
+		"file":                   "./jobsearch.db",
+		"change_batch_retention": "48h0m0s",
 	}; !reflect.DeepEqual(got, want) {
 		t.Errorf("Map = %v; want %v", got, want)
 	}
-	want := map[string]any{"database.file": "./jobsearch.db"}
+	want := map[string]any{
+		"database.file":                   "./jobsearch.db",
+		"database.change_batch_retention": "48h0m0s",
+	}
 	if got := c.Flatten(); !reflect.DeepEqual(got, want) {
 		t.Errorf("Flatten = %v; want %v", got, want)
 	}
@@ -636,6 +642,19 @@ func TestDatabaseSection(t *testing.T) {
 		conf.Database.File != "./other.db" {
 		t.Errorf("SetValue(database.file) = %v, File = %q", err, conf.Database.File)
 	}
+
+	// 30 days by default.
+	retention := defaultConfig(t).Database.ChangeBatchRetention
+	if retention != 720*time.Hour {
+		t.Errorf("default change_batch_retention = %v; want 720h", retention)
+	}
+	err := conf.SetValue("database.change_batch_retention", "0s")
+	if err != nil || conf.Database.ChangeBatchRetention != 0 {
+		t.Errorf(
+			"SetValue(change_batch_retention) = %v, retention = %v",
+			err, conf.Database.ChangeBatchRetention,
+		)
+	}
 }
 
 func TestViperValue(t *testing.T) {