Implement the database archiving target (closes #43) (#84)

Implements the `databaseTarget` as a real archiving target, replacing the always-successful stub. Delivering to a `database` target now writes the full event into a per-webhook archive SQLite file for long-term storage.

## Archive-writer semantics

- **Separate file:** each webhook's full events are written as rows into `archive-{webhookID}.db` under the data dir, distinct from the per-webhook event DB (`events-{webhookID}.db`). The file and its schema are created on first write if missing. Each row carries the full event: body, headers, method, content type, webhook id, entrypoint id, event id, and an archived-at timestamp.
- **Close/reopen with debounce:** after each write the archive handle is closed and reopened, unless the last (re)open was less than one second ago. This lets an operator move the archive file away for offline archiving while bounding file churn under load. A per-webhook `archiveWriter` owns this debounce state and serialises writes.
- **Auto-recreate:** the file is opened create-if-missing (`mode=rwc`) and its schema re-migrated on every open, so if the archive was moved or removed since the last open, the next write recreates it. The writer also detects a missing file before writing and reopens first, so a moved-away file is recreated rather than lost.
- **Optional expiry, validated at creation:** an optional `expiry` in the target's config JSON (e.g. `{"expiry":"720h"}`) is validated when the target is created (`ValidateArchiveExpiry`; bad values are rejected with a 400 at the add-target form, the Slack URL precedent). The default (missing, empty, or `"never"`) keeps rows forever with no pruning. When a positive duration is set, rows older than it (measured from each row's archived-at time) are pruned on every (re)open; because the file is reopened after writes, prune-on-open keeps the archive swept without a separate background sweeper. A set-but-invalid expiry in a stored config (unparseable, zero, or negative) is an error at delivery time too — never a silent default.
- **No-retry, fail-loud:** the target performs a single attempt with no retries. On success it records one successful attempt and marks the delivery delivered. If the archive write fails, the attempt is recorded as failed with the error and the delivery is marked failed — archiving errors never report success.

## Scope

- `internal/delivery/target_database.go` — the `databaseTarget` (no-retry) archives via a per-webhook writer registry; an archive error records a failed attempt and marks the delivery failed.
- `internal/delivery/target_database_archive.go` (new) — the `archiveWriter`, the archived-row model, config/expiry parsing (fail-loud on set-but-invalid values), `ValidateArchiveExpiry`, and prune-on-open.
- `internal/handlers/source_management.go` — database targets get a creation-validated `expiry` config (`buildDatabaseTargetConfig`); the expiry form value is read where the request body is bounded and bad values are rejected with a 400 at target creation.
- `templates/source_detail.html` — the add-target form shows an expiry field for database targets.
- `README.md` — the database-target documentation describes the archiving semantics.
- `internal/delivery/export_test.go`, `internal/delivery/target_database_test.go`, `internal/handlers` tests — tests and their exported shims.

No changes to the `Target` interface or other targets.

## Tests

- a row is archived (both at the writer level and end-to-end through `Deliver`)
- a forced archive failure (bad stored expiry config) yields a `Failed` delivery with a non-success `DeliveryResult` carrying the error and no archive file created
- the file is recreated after removal, with only the post-removal row
- the one-second reopen debounce (rapid writes reopen once; a write after the window reopens again)
- expiry pruning removes rows older than the configured expiry
- expiry config parsing (empty / `never` / duration accepted; unparseable, zero, and negative values error)
- expiry validation at target creation (`TestValidateArchiveExpiry`; valid values build the config, bad values get a 400)

## Validation

`docker build .` exits 0 (fmt-check, lint, test, build all pass).

Closes #43

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #84
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

README.md

@@ -363,10 +363,12 @@ events should be forwarded.
   greater than 0, failed deliveries are retried with exponential backoff
   up to `max_retries` attempts, protected by a per-target circuit
   breaker.
-- **`database`** — Confirm the event is stored in the webhook's
-  per-webhook database (no external delivery). Since events are always
-  written to the per-webhook DB on ingestion, this target marks delivery
-  as immediately successful. Useful for ensuring durable event archival.
+- **`database`** — Archive the full event as a row into a separate
+  per-webhook archive database (`archive-{webhookID}.db`) for long-term
+  retention, with an optional creation-validated expiry (default: keep
+  forever). No external delivery and no retries; an archive write
+  failure fails the delivery. See the database target section under
+  "Per-Webhook Event Databases" for the full semantics.
 - **`log`** — Write the event to the application log (stdout). Useful
   for debugging.
 
@@ -512,11 +514,22 @@ This separation provides:
   page cache, and its own lock, so concurrent event ingestion across
   webhooks won't contend.
 
-The **database target type** leverages this architecture: since events
-are already stored in the per-webhook database by design, the database
-target simply marks the delivery as immediately successful. The
-per-webhook DB IS the dedicated event database — that's the whole point
-of the database target type.
+The **database target type** builds on this architecture to provide
+long-term archiving, separate from the per-webhook event database (which
+may prune events under its own retention). Delivering to a database
+target writes the full event — body, headers, method, content type, and
+webhook/entrypoint/event identifiers — as a row into a dedicated archive
+database, `archive-{webhookID}.db`, stored under the data directory
+beside the event database. After each write the archive handle is closed
+and reopened, debounced to at most once per second, so an operator can
+move the archive file away for offline archiving without stopping the
+service; a moved or removed archive file is recreated automatically on
+the next write. An optional `expiry` in the target's config JSON (e.g.
+`{"expiry":"720h"}`) is validated when the target is created — the
+default (unset or the literal `never`) keeps rows forever — and rows
+older than the expiry are pruned each time the archive is (re)opened. An
+archive write failure is never silent success: the delivery records a
+failed attempt with the error and is marked failed.
 
 The **Slack target type** sends webhook events as formatted messages to
 any Slack-compatible incoming webhook URL (works with Slack, Mattermost,