feat: implement 8 OpenSpec proposals for security, reliability, and UX improvements

## Security Enhancements (P0)
- Add input validation with max_length and numeric range constraints
- Implement WebSocket token authentication via first message
- Add path traversal prevention in file storage service

## Permission Enhancements (P0)
- Add project member management for cross-department access
- Implement is_department_manager flag for workload visibility

## Cycle Detection (P0)
- Add DFS-based cycle detection for task dependencies
- Add formula field circular reference detection
- Display user-friendly cycle path visualization

## Concurrency & Reliability (P1)
- Implement optimistic locking with version field (409 Conflict on mismatch)
- Add trigger retry mechanism with exponential backoff (1s, 2s, 4s)
- Implement cascade restore for soft-deleted tasks

## Rate Limiting (P1)
- Add tiered rate limits: standard (60/min), sensitive (20/min), heavy (5/min)
- Apply rate limits to tasks, reports, attachments, and comments

## Frontend Improvements (P1)
- Add responsive sidebar with hamburger menu for mobile
- Improve touch-friendly UI with proper tap target sizes
- Complete i18n translations for all components

## Backend Reliability (P2)
- Configure database connection pool (size=10, overflow=20)
- Add Redis fallback mechanism with message queue
- Add blocker check before task deletion

## API Enhancements (P3)
- Add standardized response wrapper utility
- Add /health/ready and /health/live endpoints
- Implement project templates with status/field copying

## Tests Added
- test_input_validation.py - Schema and path traversal tests
- test_concurrency_reliability.py - Optimistic locking and retry tests
- test_backend_reliability.py - Connection pool and Redis tests
- test_api_enhancements.py - Health check and template tests

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

backend/migrations/versions/015_add_task_version_field.py

@@ -0,0 +1,29 @@
+"""Add version field to tasks for optimistic locking
+
+Revision ID: 015
+Revises: 014
+Create Date: 2026-01-10
+
+Add version integer field to tasks table for optimistic locking.
+This prevents concurrent update conflicts by tracking version numbers.
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '015'
+down_revision: Union[str, None] = '014'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add version column to pjctrl_tasks table for optimistic locking
+    op.add_column(
+        'pjctrl_tasks',
+        sa.Column('version', sa.Integer(), nullable=False, server_default='1')
+    )
+
+
+def downgrade() -> None:
+    op.drop_column('pjctrl_tasks', 'version')