Split source_management.go along its CRUD seams (closes #274)

Pure code movement. Every declaration of
internal/handlers/source_management.go moves unchanged into one of:
webhook_list.go, webhook_create.go, webhook_detail.go, webhook_edit.go
and webhook_delete.go for the webhook pages; event_log.go for the event
log; entrypoint.go for the entrypoint handlers; target_create.go,
target_delete.go and target_toggle.go for the target handlers; and
shared.go for the helpers several of them use. Only each file's package
line and imports are new. The README and a middleware comment that
named the removed file now name the new ones.

Model: opus-5-5

README.md

@@ -2840,8 +2840,9 @@ read as more than it is:
 
 - **Lines carrying an authenticated operator's own input**, which are
   not truncated at all. `webhook created` logs the submitted `name`
-  verbatim and `target URL blocked by SSRF protection` logs the target
-  host (both `internal/handlers/source_management.go`), as do the
+  verbatim (`internal/handlers/webhook_create.go`) and
+  `target URL blocked by SSRF protection` logs the target host
+  (`internal/handlers/target_create.go`), as do the
   `target_name` lines in `internal/delivery/engine.go` and
   `internal/delivery/target_http.go`. The only bound on any of them is
   the 1 MB form body cap, so a 100 KB `name` writes a single line of
@@ -3220,7 +3221,17 @@ webhooker/
 │   │   ├── index.go           # Index page handler
 │   │   ├── profile.go         # User profile handler
 │   │   ├── settings.go        # Read-only Settings page handler
-│   │   ├── source_management.go  # Webhook CRUD handlers
+│   │   ├── webhook_list.go    # Webhook list page
+│   │   ├── webhook_create.go  # Webhook create
+│   │   ├── webhook_detail.go  # Webhook detail page
+│   │   ├── webhook_edit.go    # Webhook edit, archive renaming
+│   │   ├── webhook_delete.go  # Webhook delete, event database and archive writer removal
+│   │   ├── event_log.go       # Event log page: loaders, filters, delivery views
+│   │   ├── entrypoint.go      # Entrypoint create, edit, delete and toggle
+│   │   ├── target_create.go   # Target create, per-type config builders
+│   │   ├── target_delete.go   # Target delete
+│   │   ├── target_toggle.go   # Target toggle
+│   │   ├── shared.go          # Helpers shared by several handlers
 │   │   └── webhook.go         # Webhook receiver handler
 │   ├── healthcheck/
 │   │   └── healthcheck.go     # Health check service (uptime, version)