feat(vocab): implement SRS features and enhance vocabulary management

- Added new endpoints in vocabController for retrieving SRS due items and reviewing SRS items, improving spaced repetition support.
- Updated vocabService to handle SRS item creation and scheduling, ensuring effective tracking of vocabulary exposure.
- Enhanced vocabRouter with new routes for SRS functionalities, facilitating user interaction with spaced repetition features.
- Modified VocabPracticeDialog and VocabCourseView to integrate SRS due items, providing users with timely review opportunities.
- Updated translations and UI elements to reflect new SRS features, enhancing user experience and accessibility.

backend/sql/add_vocab_srs_item.sql

@@ -0,0 +1,35 @@
+CREATE TABLE IF NOT EXISTS community.vocab_srs_item (
+  id SERIAL PRIMARY KEY,
+  user_id INTEGER NOT NULL REFERENCES community."user"(id) ON DELETE CASCADE,
+  course_id INTEGER NOT NULL REFERENCES community.vocab_course(id) ON DELETE CASCADE,
+  lesson_id INTEGER NULL REFERENCES community.vocab_course_lesson(id) ON DELETE SET NULL,
+  item_key VARCHAR(80) NOT NULL,
+  learning TEXT NOT NULL,
+  reference TEXT NOT NULL,
+  direction VARCHAR(8) NOT NULL DEFAULT 'BOTH',
+  stage INTEGER NOT NULL DEFAULT 0,
+  interval_days INTEGER NOT NULL DEFAULT 0,
+  last_reviewed_at TIMESTAMP WITH TIME ZONE NULL,
+  next_due_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
+  correct_count INTEGER NOT NULL DEFAULT 0,
+  wrong_count INTEGER NOT NULL DEFAULT 0,
+  lapse_count INTEGER NOT NULL DEFAULT 0,
+  created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW(),
+  CONSTRAINT vocab_srs_item_user_key_unique UNIQUE (user_id, item_key)
+);
+
+CREATE INDEX IF NOT EXISTS idx_vocab_srs_item_due
+  ON community.vocab_srs_item (user_id, course_id, next_due_at);
+
+CREATE INDEX IF NOT EXISTS idx_vocab_srs_item_lesson
+  ON community.vocab_srs_item (user_id, course_id, lesson_id);
+
+COMMENT ON TABLE community.vocab_srs_item IS
+  'Nutzerbezogener SRS-Fortschritt pro Vokabel/Phrase aus Sprachkursen.';
+COMMENT ON COLUMN community.vocab_srs_item.item_key IS
+  'Stabiler deterministischer Schlüssel aus Kurs, Lektion und normalisiertem Begriffspaar.';
+COMMENT ON COLUMN community.vocab_srs_item.stage IS
+  'SRS-Stufe. Höhere Stufen bedeuten längere Wiederholungsintervalle.';
+COMMENT ON COLUMN community.vocab_srs_item.next_due_at IS
+  'Zeitpunkt, zu dem das Item wieder fällig ist.';