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