Add a Download button that exports a database target's archive as gzipped JSON (closes #374)

Each database target on the webhook page links to
/hook/ID/targets/TARGETID/download, which streams the target's archive
as archive-WEBHOOK-TARGET-YYYYMMDDTHHMMSSZ.json.gz: the webhook, the
target, exported_at, and archived_events, one object per row keyed by
column, a body that is not valid UTF-8 in base64 with body_encoding.
The export reads the rows through one cursor on its own connection
inside a read-only transaction: one snapshot, and no write lock. A
download runs for as long as the client keeps reading, and one that
fails after it has started aborts the connection. The handler holds the
rename lock only while it reads the names and opens the file. A missing
archive exports empty and is not created.

Model: opus-5-5

README.md

@@ -2029,6 +2029,29 @@ Because each `database` target has its own archive file, a target's
 webhook with different expiries keep two archives, each pruned on its
 own schedule.
 
+Each `database` target on the webhook page has a **Download** button,
+which returns its archive as one gzipped JSON file,
+`archive-{webhook_name}-{target_name}-{YYYYMMDDTHHMMSSZ}.json.gz`, the
+names made safe as above and the time in UTC. The file holds one
+object: `webhook` and `target`, each an `id` and a `name`;
+`exported_at`; and `archived_events`, one object per archived row with
+every column, keyed by column name. A body that is not valid UTF-8 is
+written in base64, with `"body_encoding": "base64"` beside it. An
+archive that does not exist yet, or was moved away, downloads with an
+empty `archived_events`; the download never creates the file.
+
+The download streams: each row is read and written out compressed
+before the next is read, so neither the archive nor the JSON is held in
+memory. It reads on a connection of its own, inside one read-only
+transaction, so the file holds the archive as it stood when the
+download started, and archive writes go on meanwhile, since under WAL a
+reader never blocks a writer. While it runs, the `-wal` cannot be
+checkpointed past what it reads, so a long download lets the `-wal`
+grow. It finds the file by the stored names under the lock that webhook
+edits, target edits and target creation hold, and lets go once the file
+is open: a rename during the download moves the file without affecting
+it.
+
 Deleting a webhook releases its archives: the delivery engine's cached
 archive writers are dropped and their file handles closed, so nothing
 lingers after the webhook is gone. The archive **files themselves are
@@ -2892,6 +2915,7 @@ returns to the page that was asked for.
 | `POST` | `/hook/{id}/targets`     | Add target to webhook |
 | `GET`  | `/hook/{id}/targets/{targetID}/edit` | Edit target form. The one page that renders a target's destination URL and header values in full, rather than masked |
 | `POST` | `/hook/{id}/targets/{targetID}/edit` | Edit target submission |
+| `GET`  | `/hook/{id}/targets/{targetID}/download` | Download a `database` target's archive as one gzipped JSON file. See [Database Architecture](#database-architecture) |
 | `POST` | `/hook/{id}/targets/{targetID}/delete` | Delete a target |
 | `POST` | `/hook/{id}/targets/{targetID}/toggle` | Enable or disable a target |
 
@@ -2972,6 +2996,7 @@ webhooker/
 │   │   ├── target_slack.go    # Slack/Mattermost incoming-webhook target
 │   │   ├── target_database.go # Database archive target
 │   │   ├── target_database_archive.go  # Archive file lifecycle and pruning
+│   │   ├── target_database_export.go   # Archive download as gzipped JSON
 │   │   ├── target_log.go      # Log target (stdout)
 │   │   ├── target_config_view.go  # Masked target config for templates
 │   │   ├── archive_sweeper.go # Periodic pruning of idle archives