Add comprehensive stats endpoints and reporting features

Backend stats endpoints:
- GET /api/v1/project/:project/packages/:package/stats - per-package stats
- GET /api/v1/artifact/:id/stats - artifact reference statistics
- GET /api/v1/stats/cross-project - cross-project deduplication detection
- GET /api/v1/stats/timeline - time-based metrics (daily/weekly/monthly)
- GET /api/v1/stats/export - CSV/JSON export
- GET /api/v1/stats/report - markdown/JSON summary report generation

Enhanced existing endpoints:
- Added storage_saved_bytes and deduplication_ratio to project stats
- Added date range filtering via from_date/to_date params

New schemas:
- PackageStatsResponse
- ArtifactStatsResponse
- CrossProjectDeduplicationResponse
- TimeBasedStatsResponse
- StatsReportResponse

backend/app/schemas.py

@@ -456,3 +456,62 @@ class ProjectStatsResponse(BaseModel):
     total_size_bytes: int
     upload_count: int
     deduplicated_uploads: int
+    storage_saved_bytes: int = 0  # Bytes saved through deduplication
+    deduplication_ratio: float = 1.0  # upload_count / artifact_count
+
+
+class PackageStatsResponse(BaseModel):
+    """Per-package statistics"""
+
+    package_id: str
+    package_name: str
+    project_name: str
+    tag_count: int
+    artifact_count: int
+    total_size_bytes: int
+    upload_count: int
+    deduplicated_uploads: int
+    storage_saved_bytes: int = 0
+    deduplication_ratio: float = 1.0
+
+
+class ArtifactStatsResponse(BaseModel):
+    """Per-artifact reference statistics"""
+
+    artifact_id: str
+    sha256: str
+    size: int
+    ref_count: int
+    storage_savings: int  # (ref_count - 1) * size
+    tags: List[Dict[str, Any]]  # Tags referencing this artifact
+    projects: List[str]  # Projects using this artifact
+    packages: List[str]  # Packages using this artifact
+    first_uploaded: Optional[datetime] = None
+    last_referenced: Optional[datetime] = None
+
+
+class CrossProjectDeduplicationResponse(BaseModel):
+    """Cross-project deduplication statistics"""
+
+    shared_artifacts_count: int  # Artifacts used in multiple projects
+    total_cross_project_savings: int  # Bytes saved by cross-project sharing
+    shared_artifacts: List[Dict[str, Any]]  # Details of shared artifacts
+
+
+class TimeBasedStatsResponse(BaseModel):
+    """Time-based deduplication statistics"""
+
+    period: str  # "daily", "weekly", "monthly"
+    start_date: datetime
+    end_date: datetime
+    data_points: List[
+        Dict[str, Any]
+    ]  # List of {date, uploads, unique, duplicated, bytes_saved}
+
+
+class StatsReportResponse(BaseModel):
+    """Summary report in various formats"""
+
+    format: str  # "json", "csv", "markdown"
+    generated_at: datetime
+    content: str  # The report content