Add Feedback domain module: public submission flow, admin CRUD, reporting (#7)

Co-authored-by: Patrick Müller <mail@pmueller.me>
Reviewed-on: #7
Co-authored-by: Patrick Mueller <patrick@mueller-patrick.tech>
Co-committed-by: Patrick Mueller <patrick@mueller-patrick.tech>

sql/feedback/002_add_poster_image_url.sql

@@ -0,0 +1,9 @@
+-- Nachklang e.V. Feedback module — adds a per-event poster image URL.
+-- Apply manually against the FEEDBACK_DB database, after 001_init.sql:
+--   mysql -h <DB_HOST> -u <DB_USER> -p <FEEDBACK_DB> < 002_add_poster_image_url.sql
+--
+-- Stores a URL only (e.g. an existing nachklang.art poster image) rather
+-- than an uploaded file — the concert posters already live on the public
+-- website, so there is no need for the feedback app to host its own copy.
+ALTER TABLE events
+    ADD COLUMN poster_image_url VARCHAR(500) NULL AFTER intro_text;