Give both summaries a modified time, and the dashboard list a footprint

Home's "recently modified" order was a proxy — drafts first, then the
version counter, then the name. Both summaries now carry `updated_at`,
read as the mtime of the working copy: every write in the store commits
immediately, so a file's mtime is its commit time, and `git log -1 --
<path>` costs ~990ms across this instance's 15 documents (it walks the
history back to the last commit touching each one, so it is slowest for
the stalest) against ~1.8ms for the stats. No cache needed.

`DashboardSummary` also carries `footprint`: each widget as its type plus
the placement a panel resolves, which is the whole of what the mosaic
draws. That drops the document fetch per tile and with it the cap of
eight, past which tiles showed a name and nothing else. Verified against
this instance's 8 dashboards: the blocks are identical to what the old
client-side derivation produced.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CL9zvnnvcp1mvA8o7impxk

backend/fluksio/flow/schemas.py

@@ -7,6 +7,7 @@ A flow is structure plus code: this module is the structure. Node logic for
 from __future__ import annotations
 
 import re
+from datetime import datetime
 from typing import Any, Literal
 
 from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator
@@ -366,6 +367,9 @@ class FlowSummary(BaseModel):
     quarantined: bool = False
     #: Of the working copy, so publishing from a list needs no second read.
     version: int = 1
+    #: When the working copy was last written, so a list can be ordered by
+    #: what was worked on rather than by how often it has been saved.
+    updated_at: datetime | None = None
 
 
 class FlowsPublic(BaseModel):