feat: add skip status tracking for job orchestration

Implement skip status tracking to fix jobs hanging when dates are
filtered out. Jobs now properly complete when all model-days reach
terminal states (completed/failed/skipped).

Changes:
- database.py: Add 'skipped' status to job_details CHECK constraint
- job_manager.py: Update completion logic to count skipped as done
- job_manager.py: Add skipped count to progress tracking
- simulation_worker.py: Implement skip tracking with per-model granularity
- simulation_worker.py: Add _filter_completed_dates_with_tracking()
- simulation_worker.py: Add _mark_skipped_dates()
- simulation_worker.py: Update _prepare_data() to use skip tracking
- simulation_worker.py: Improve warning messages to distinguish skip types

Skip reasons:
- "Already completed" - Position data exists from previous job
- "Incomplete price data" - Missing prices (weekends/holidays/future)

The implementation correctly handles multi-model scenarios where different
models have different completion states for the same date.

api/database.py

@@ -105,7 +105,7 @@ def initialize_database(db_path: str = "data/jobs.db") -> None:
             job_id TEXT NOT NULL,
             date TEXT NOT NULL,
             model TEXT NOT NULL,
-            status TEXT NOT NULL CHECK(status IN ('pending', 'running', 'completed', 'failed')),
+            status TEXT NOT NULL CHECK(status IN ('pending', 'running', 'completed', 'failed', 'skipped')),
             started_at TEXT,
             completed_at TEXT,
             duration_seconds REAL,