Add custom emoji mapping system for premium emoji support

- Create emoji_mappings table to store emoji->emoji_id mappings
- Add EmojiMappingService for managing emoji registration and replacement
- Add admin emoji handlers (/add_emoji, /my_emojis, /delete_emoji, /all_emojis)
- Create emoji message helper for automatic emoji processing
- Add Alembic migration for emoji_mappings table
- Integrate emoji router into main dispatcher
- Add comprehensive documentation (EMOJI_SYSTEM.md)
- Fix migration chain issue with merge_migration

Features:
- Admins can register premium emojis via /add_emoji command
- Automatic emoji->emoji_id replacement before sending messages
- Per-admin unique constraint on emoji registration
- Track last used timestamp for analytics
- Bulk operations support

migrations/versions/20260218_0402_12_merge_migration.py

@@ -1,7 +1,7 @@
 """merge branches
 
 Revision ID: merge_migration
-Revises: 41aae82e631b, cd31303a681c
+Revises: cd31303a681c
 Create Date: 2026-02-18 04:02:12.000000
 
 """
@@ -11,7 +11,7 @@ import sqlalchemy as sa
 
 # revision identifiers, used by Alembic.
 revision = 'merge_migration'
-down_revision = ('41aae82e631b', 'cd31303a681c')
+down_revision = 'cd31303a681c'
 branch_labels = None
 depends_on = None
 

migrations/versions/20260307_0100_add_emoji_mappings.py

@@ -0,0 +1,45 @@
+"""Add emoji_mappings table for storing custom emoji IDs
+
+Revision ID: 20260307_0100_add_emoji_mappings
+Revises: merge_migration
+Create Date: 2026-03-07 01:00:00.000000
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '20260307_0100_add_emoji_mappings'
+down_revision = 'merge_migration'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table(
+        'emoji_mappings',
+        sa.Column('id', sa.Integer(), nullable=False),
+        sa.Column('emoji_text', sa.String(length=10), nullable=False),
+        sa.Column('emoji_id', sa.String(length=255), nullable=False),
+        sa.Column('admin_id', sa.Integer(), nullable=False),
+        sa.Column('description', sa.String(length=255), nullable=True),
+        sa.Column('created_at', sa.DateTime(timezone=True), nullable=False),
+        sa.Column('last_used_at', sa.DateTime(timezone=True), nullable=True),
+        sa.ForeignKeyConstraint(['admin_id'], ['users.id'], ),
+        sa.PrimaryKeyConstraint('id'),
+        sa.UniqueConstraint('emoji_id', name='emoji_mappings_emoji_id_key'),
+        sa.UniqueConstraint('emoji_text', 'admin_id', name='unique_emoji_per_admin'),
+    )
+    op.create_index('ix_emoji_mappings_emoji_id', 'emoji_mappings', ['emoji_id'], unique=True)
+    op.create_index('ix_emoji_mappings_emoji_text', 'emoji_mappings', ['emoji_text'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index('ix_emoji_mappings_emoji_text', table_name='emoji_mappings')
+    op.drop_index('ix_emoji_mappings_emoji_id', table_name='emoji_mappings')
+    op.drop_table('emoji_mappings')
+    # ### end Alembic commands ###