Align docs with archive semantics; fail loud on non-positive expiry (#43)

- README: rewrite the database-target documentation (target-types
  bullet and the per-webhook databases section) to describe the
  shipped archiving semantics -- separate archive-{webhookID}.db,
  debounced close/reopen for offline archiving, auto-recreate,
  creation-validated optional expiry with prune-on-open, and
  fail-loud delivery on archive write errors -- replacing the
  stale always-successful stub description.
- parseArchiveExpiry now returns an error for set-but-non-positive
  durations ("0s", "-5h") instead of silently defaulting to
  keep-forever, matching ValidateArchiveExpiry at creation time;
  the delivery then fails loudly like any other archive error.
  TestParseArchiveExpiry extended with zero and negative cases.
- databaseTarget type comment: "fire-and-forget" -> "no-retry",
  matching the fail-loud behaviour.

internal/delivery/target_database.go

@@ -10,7 +10,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/database"
 )
 
-// databaseTarget is a fire-and-forget target that archives the
+// databaseTarget is a no-retry target that archives the
 // full inbound event into a per-webhook archive SQLite file,
 // separate from the per-webhook event database. The event is
 // already persisted in the per-webhook event DB by the time