Read form fields from the POST body only (closes #160)

internal/handlers/source_management.go read the target destination
with r.FormValue, which falls back to the URL query string when the
field is absent from the body. So

    POST /source/{id}/targets?url=https://hooks.slack.com/services/T/B/S

created a working target from a value carried on the request line,
where logs, proxies, Referer headers and error trackers record it.
That is the remaining ingress path of the credential-exposure class
the render, delivery-error and log-line paths were each closed for.

Every form read in these handlers is now r.PostFormValue, so no
query-string value can populate stored configuration or be taken as a
credential. The one deliberate query read, `page` on the authenticated
pagination links, is untouched: it uses r.URL.Query().Get already.

The access log no longer carries the query on any branch, so the log
half of the report is already mitigated; the Sentry half is not. The
SDK attaches the request to every captured event and copies
r.URL.RawQuery into Request.QueryString independently of the access
log, so a BeforeSend hook clears that field before an event leaves the
process. Scheme, host, path and method stay, which is what names the
failing route.

Second barrier, for the JSON path that does not exist yet: the fields
that hold a credential are tagged json:"-" so the first handler to
marshal a model cannot serialise one. Target.Config holds the
incoming-webhook URL, APIKey.Key is a bearer token, and Setting.Value
holds the session encryption key. delivery.TargetView remains the
masking barrier for the HTML path, which is unaffected.

internal/database/model_setting.go

@@ -3,6 +3,9 @@ package database
 // Setting stores application-level key-value configuration.
 // Used for auto-generated values like the session encryption key.
 type Setting struct {
-	Key   string `gorm:"primaryKey"         json:"key"`
-	Value string `gorm:"type:text;not null" json:"value"`
+	Key string `gorm:"primaryKey" json:"key"`
+
+	// Value holds the session encryption key, so it is never
+	// marshalled with the model.
+	Value string `gorm:"type:text;not null" json:"-"`
 }