feat(sharing): add document sharing with expiring links

- Add SharedLink model with token, expiry, view limit, password hash
- Add migration 025_add_shared_links
- Add API endpoints: create, list, revoke (auth) + public info/download
- Add management UI at /shared-links with revoke controls
- Add public share landing page at /share/{token}
- Add Share button on file_view.html
- Add Shared Links to user dropdown in common.js
- Write 35 unit tests covering all scenarios
- Update UserGuide.md with sharing documentation

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

migrations/versions/025_add_shared_links.py

@@ -0,0 +1,50 @@
+"""Add shared_links table for document sharing with expiring links
+
+Revision ID: 025_add_shared_links
+Revises: 024_add_api_tokens
+Create Date: 2026-03-08
+"""
+
+from typing import Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "025_add_shared_links"
+down_revision: Union[str, None] = "024_add_api_tokens"
+depends_on: Union[str, None] = None
+
+
+def upgrade() -> None:
+    """Create shared_links table."""
+    op.create_table(
+        "shared_links",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("token", sa.String(64), nullable=False),
+        sa.Column("file_id", sa.Integer(), nullable=False),
+        sa.Column("owner_id", sa.String(), nullable=False),
+        sa.Column("label", sa.String(255), nullable=True),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("max_views", sa.Integer(), nullable=True),
+        sa.Column("view_count", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("password_hash", sa.String(128), nullable=True),
+        sa.Column("is_active", sa.Boolean(), nullable=False, server_default="1"),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now()),
+        sa.Column("revoked_at", sa.DateTime(timezone=True), nullable=True),
+        sa.ForeignKeyConstraint(["file_id"], ["files.id"]),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("token"),
+    )
+    op.create_index("ix_shared_links_id", "shared_links", ["id"])
+    op.create_index("ix_shared_links_token", "shared_links", ["token"])
+    op.create_index("ix_shared_links_file_id", "shared_links", ["file_id"])
+    op.create_index("ix_shared_links_owner_id", "shared_links", ["owner_id"])
+
+
+def downgrade() -> None:
+    """Drop shared_links table."""
+    op.drop_index("ix_shared_links_owner_id", "shared_links")
+    op.drop_index("ix_shared_links_file_id", "shared_links")
+    op.drop_index("ix_shared_links_token", "shared_links")
+    op.drop_index("ix_shared_links_id", "shared_links")
+    op.drop_table("shared_links")