feat: period-scoped holiday management, archive lifecycle, and docs/release sync

- add period-scoped holiday architecture end-to-end
	- model: scope `SchoolHoliday` to `academic_period_id`
	- migrations: add holiday-period scoping, academic-period archive lifecycle, and merge migration head
	- API: extend holidays with manual CRUD, period validation, duplicate prevention, and overlap merge/conflict handling
	- recurrence: regenerate holiday exceptions using period-scoped holiday sets

- improve frontend settings and holiday workflows
	- bind holiday import/list/manual CRUD to selected academic period
	- show detailed import outcomes (inserted/updated/merged/skipped/conflicts)
	- fix file-picker UX (visible selected filename)
	- align settings controls/dialogs with defined frontend design rules
	- scope appointments/dashboard holiday loading to active period
	- add shared date formatting utility

- strengthen academic period lifecycle handling
	- add archive/restore/delete flow and backend validations/blocker checks
	- extend API client support for lifecycle operations

- release/docs updates and cleanup
	- bump user-facing version to `2026.1.0-alpha.15` with new changelog entry
	- add tech changelog entry for alpha.15 backend changes
	- refactor README to concise index and archive historical implementation docs
	- fix Copilot instruction link diagnostics via local `.github` design-rules reference

server/alembic/versions/a7b8c9d0e1f2_add_archive_lifecycle_to_academic_periods.py

@@ -0,0 +1,55 @@
+"""Add archive lifecycle fields to academic_periods
+
+Revision ID: a7b8c9d0e1f2
+Revises: 910951fd300a
+Create Date: 2026-03-31 00:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a7b8c9d0e1f2'
+down_revision: Union[str, None] = '910951fd300a'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # Add archive lifecycle fields to academic_periods table
+    op.add_column('academic_periods', sa.Column('is_archived', sa.Boolean(), nullable=False, server_default='0'))
+    op.add_column('academic_periods', sa.Column('archived_at', sa.TIMESTAMP(timezone=True), nullable=True))
+    op.add_column('academic_periods', sa.Column('archived_by', sa.Integer(), nullable=True))
+    
+    # Add foreign key for archived_by
+    op.create_foreign_key(
+        'fk_academic_periods_archived_by_users_id',
+        'academic_periods',
+        'users',
+        ['archived_by'],
+        ['id'],
+        ondelete='SET NULL'
+    )
+    
+    # Add indexes for performance
+    op.create_index('ix_academic_periods_archived', 'academic_periods', ['is_archived'])
+    op.create_index('ix_academic_periods_name_not_archived', 'academic_periods', ['name', 'is_archived'])
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # Drop indexes
+    op.drop_index('ix_academic_periods_name_not_archived', 'academic_periods')
+    op.drop_index('ix_academic_periods_archived', 'academic_periods')
+    
+    # Drop foreign key
+    op.drop_constraint('fk_academic_periods_archived_by_users_id', 'academic_periods')
+    
+    # Drop columns
+    op.drop_column('academic_periods', 'archived_by')
+    op.drop_column('academic_periods', 'archived_at')
+    op.drop_column('academic_periods', 'is_archived')

server/alembic/versions/dd100f3958dc_merge_academic_periods_and_client_.py

@@ -0,0 +1,28 @@
+"""merge academic periods and client monitoring heads
+
+Revision ID: dd100f3958dc
+Revises: a7b8c9d0e1f2, c1d2e3f4g5h6
+Create Date: 2026-03-31 07:55:09.999917
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'dd100f3958dc'
+down_revision: Union[str, None] = ('a7b8c9d0e1f2', 'c1d2e3f4g5h6')
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    pass
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    pass

server/alembic/versions/f3c4d5e6a7b8_scope_school_holidays_to_academic_.py

@@ -0,0 +1,54 @@
+"""scope school holidays to academic periods
+
+Revision ID: f3c4d5e6a7b8
+Revises: dd100f3958dc
+Create Date: 2026-03-31 12:20:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f3c4d5e6a7b8'
+down_revision: Union[str, None] = 'dd100f3958dc'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('school_holidays', sa.Column('academic_period_id', sa.Integer(), nullable=True))
+    op.create_index(
+        op.f('ix_school_holidays_academic_period_id'),
+        'school_holidays',
+        ['academic_period_id'],
+        unique=False,
+    )
+    op.create_foreign_key(
+        'fk_school_holidays_academic_period_id',
+        'school_holidays',
+        'academic_periods',
+        ['academic_period_id'],
+        ['id'],
+        ondelete='SET NULL',
+    )
+    op.drop_constraint('uq_school_holidays_unique', 'school_holidays', type_='unique')
+    op.create_unique_constraint(
+        'uq_school_holidays_unique',
+        'school_holidays',
+        ['name', 'start_date', 'end_date', 'region', 'academic_period_id'],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint('uq_school_holidays_unique', 'school_holidays', type_='unique')
+    op.create_unique_constraint(
+        'uq_school_holidays_unique',
+        'school_holidays',
+        ['name', 'start_date', 'end_date', 'region'],
+    )
+    op.drop_constraint('fk_school_holidays_academic_period_id', 'school_holidays', type_='foreignkey')
+    op.drop_index(op.f('ix_school_holidays_academic_period_id'), table_name='school_holidays')
+    op.drop_column('school_holidays', 'academic_period_id')