feat(tournament): implement multi-stage tournament support with intermediate and final stages

- Added backend controller for tournament stages with endpoints to get, upsert, and advance stages.
- Created database migration for new tables: tournament_stage and tournament_stage_advancement.
- Updated models for TournamentStage and TournamentStageAdvancement.
- Enhanced frontend components to manage tournament stages, including configuration for intermediate and final rounds.
- Implemented logic for saving and advancing tournament stages, including handling of pool rules and third place matches.
- Added error handling and loading states in the frontend for better user experience.

backend/migrations/20251213_add_tournament_stages.sql

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+-- Adds multi-stage tournaments (rounds) support
+-- MariaDB/MySQL compatible migration (manual execution)
+
+-- 1) New table: tournament_stage
+CREATE TABLE IF NOT EXISTS tournament_stage (
+  id INT NOT NULL AUTO_INCREMENT,
+  tournament_id INT NOT NULL,
+  stage_index INT NOT NULL,
+  name VARCHAR(255) NULL,
+  type VARCHAR(32) NOT NULL, -- 'groups' | 'knockout'
+  number_of_groups INT NULL,
+  advancing_per_group INT NULL,
+  max_group_size INT NULL,
+  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  PRIMARY KEY (id),
+  CONSTRAINT fk_tournament_stage_tournament
+    FOREIGN KEY (tournament_id) REFERENCES tournament(id)
+    ON DELETE CASCADE
+) ENGINE=InnoDB;
+
+CREATE INDEX idx_tournament_stage_tournament_id ON tournament_stage (tournament_id);
+CREATE UNIQUE INDEX uq_tournament_stage_tournament_id_index ON tournament_stage (tournament_id, stage_index);
+
+-- 2) New table: tournament_stage_advancement
+CREATE TABLE IF NOT EXISTS tournament_stage_advancement (
+  id INT NOT NULL AUTO_INCREMENT,
+  tournament_id INT NOT NULL,
+  from_stage_id INT NOT NULL,
+  to_stage_id INT NOT NULL,
+  mode VARCHAR(32) NOT NULL DEFAULT 'pools',
+  config JSON NOT NULL,
+  created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  PRIMARY KEY (id),
+  CONSTRAINT fk_tournament_stage_adv_tournament
+    FOREIGN KEY (tournament_id) REFERENCES tournament(id)
+    ON DELETE CASCADE,
+  CONSTRAINT fk_tournament_stage_adv_from
+    FOREIGN KEY (from_stage_id) REFERENCES tournament_stage(id)
+    ON DELETE CASCADE,
+  CONSTRAINT fk_tournament_stage_adv_to
+    FOREIGN KEY (to_stage_id) REFERENCES tournament_stage(id)
+    ON DELETE CASCADE
+) ENGINE=InnoDB;
+
+CREATE INDEX idx_tournament_stage_adv_tournament_id ON tournament_stage_advancement (tournament_id);
+CREATE INDEX idx_tournament_stage_adv_from_stage_id ON tournament_stage_advancement (from_stage_id);
+CREATE INDEX idx_tournament_stage_adv_to_stage_id ON tournament_stage_advancement (to_stage_id);
+
+-- 3) Add stage_id to tournament_group and tournament_match
+-- MariaDB has no IF NOT EXISTS for columns; run each ALTER once.
+-- If you rerun, comment out the ALTERs or check INFORMATION_SCHEMA first.
+ALTER TABLE tournament_group ADD COLUMN stage_id INT NULL;
+ALTER TABLE tournament_match ADD COLUMN stage_id INT NULL;
+
+CREATE INDEX idx_tournament_group_tournament_stage ON tournament_group (tournament_id, stage_id);
+CREATE INDEX idx_tournament_match_tournament_stage ON tournament_match (tournament_id, stage_id);

backend/migrations/20251214_allow_null_players_in_tournament_match.sql

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+-- Allow NULL placeholders for KO (e.g. "Spiel um Platz 3")
+-- MariaDB/MySQL manual migration
+--
+-- Background: We create placeholder matches with player1_id/player2_id = NULL.
+-- Some prod DBs still have NOT NULL on these columns.
+
+-- 1) Make player columns nullable
+ALTER TABLE tournament_match MODIFY COLUMN player1_id INT NULL;
+ALTER TABLE tournament_match MODIFY COLUMN player2_id INT NULL;
+
+-- 2) (Optional) If you have foreign keys to tournament_member/external participant IDs,
+-- ensure they also allow NULL. (Not adding here because not all installations have FKs.)
+
+-- 3) Verify
+-- SHOW COLUMNS FROM tournament_match LIKE 'player1_id';
+-- SHOW COLUMNS FROM tournament_match LIKE 'player2_id';