Gitea setup: Core Rule Set blocks attacks, not ordinary Gitea traffic (closes #198)

The Gitea setup runs the Core Rule Set in block mode. Rules that refused
ordinary Gitea requests are switched off only where they misfired:

- deploy/gitea/crs-exclusions.conf, loaded through the new setting
  SWWAF_CORE_RULE_SET_EXCLUSIONS_FILE before the Core Rule Set's rules,
  covers Gitea's search, file-name and branch-name fields, Referer, file
  paths, API attachment names and Actions artifact uploads;
- smallwebwaf's defaults gain a seventh change: the registry upload
  path's from parameter skips 932260.

A file that cannot be read, does not parse or names an id from 900000
to 900999 stops servewaf at start, in every mode. internal/waf's tests
and script/gitea-setup send the recorded requests in
internal/waf/testdata/gitea-requests.txt and three attacks.

Model: opus-5-5

docs/settings.md

@@ -192,6 +192,13 @@ chooses between the two (see "The image's command" below).
   the Core Rule Set reads, a size of at most `64K`. A question carries no more
   of a body than 64 KiB, and the Core Rule Set reads at most 64 KiB less one
   byte of it, whatever the value (see "The answer" in [`SPEC.md`](../SPEC.md)).
+- `SWWAF_CORE_RULE_SET_EXCLUSIONS_FILE` (default unset): a file of Core Rule Set
+  exclusions, written as Coraza directives. It is loaded before the Core Rule
+  Set's rules, so it holds only `SecAction` and `SecRule` lines that exclude
+  with `ctl:ruleRemoveTargetById` or `ctl:ruleRemoveById`. A file that cannot be
+  read or does not parse stops the start, whatever `SWWAF_WAF_MODE` is. So does
+  a file that names a number from 900000 to 900999 outside a comment, alone or
+  in a range. It has no `_FILE` form.
 - `SWWAF_ERROR_BURST_THRESHOLD` (default `30`): the most refusals of one client
   in a minute (see "Attack detection" in `SPEC.md`). A count or `off`.