feat(phase2): Add achievements and replay systems

Achievement System:
- Add Achievement model with condition types (streak, steal, specialist, etc.)
- Add AchievementManager service for tracking and awarding achievements
- Add Pydantic schemas for achievements (AchievementResponse, PlayerStats, etc.)
- Seed 18 achievements from design doc
- Add GET /api/game/achievements endpoint

Replay System:
- Add ReplayManager service for saving/loading game replays
- Add GET /api/replay/{code} and /api/replay/session/{id} endpoints
- Format replays for frontend consumption

Phase 2 tasks completed:
- F2.1: Achievement model and migration
- F2.2: Pydantic schemas
- F2.3: AchievementManager service
- F2.4: ReplayManager service
- F2.5: API endpoints
- F2.6: Seed 18 achievements data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/alembic/env.py

@@ -8,7 +8,7 @@ import os
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 from app.models.base import Base
-from app.models import Category, Question, GameSession, GameEvent, Admin
+from app.models import Category, Question, GameSession, GameEvent, Admin, Achievement
 
 config = context.config
 if config.config_file_name is not None:

backend/alembic/versions/f207a5a45cfa_add_achievements_table.py

@@ -0,0 +1,44 @@
+"""add_achievements_table
+
+Revision ID: f207a5a45cfa
+Revises: 65d30b7402cf
+Create Date: 2026-01-26 08:14:59.697355
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f207a5a45cfa'
+down_revision: Union[str, None] = '65d30b7402cf'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('achievements',
+    sa.Column('id', sa.Integer(), nullable=False),
+    sa.Column('name', sa.String(length=100), nullable=False),
+    sa.Column('description', sa.Text(), nullable=False),
+    sa.Column('icon', sa.String(length=10), nullable=False),
+    sa.Column('condition_type', sa.String(length=50), nullable=False),
+    sa.Column('condition_value', sa.Integer(), nullable=True),
+    sa.Column('category_id', sa.Integer(), nullable=True),
+    sa.Column('created_at', sa.DateTime(), server_default=sa.text('now()'), nullable=True),
+    sa.ForeignKeyConstraint(['category_id'], ['categories.id'], ),
+    sa.PrimaryKeyConstraint('id'),
+    sa.UniqueConstraint('name')
+    )
+    op.create_index(op.f('ix_achievements_id'), 'achievements', ['id'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_achievements_id'), table_name='achievements')
+    op.drop_table('achievements')
+    # ### end Alembic commands ###