fix(dc_bridge): carry Measurement custom keys onto File metadata Records

[Minipada]

Closes #419

Problem

A Measurement's custom_key_str_list keys are appended to the JSON of its Records, so they
reach a PostgreSQL Destination as columns. The Uploader built its file_status and
group_complete rows from a fixed field set, so the same keys were silently dropped on
the File side: configure custom_key_str_list: ["site"] on a camera Measurement and site
appeared in dc_records and never in dc_files, with nothing logged.

What changed

The Record names its custom keys. Measurement::addCustomKeys() now writes a
custom_keys array alongside the values it already inlines. Without it the Uploader has no
way to tell site from a measured field — every top-level key of a Record looks the same
from the Bridge. A Measurement with no custom keys writes nothing, exactly as before.

The Uploader carries them onto every File metadata row. parse_file_group() reads the
declaration and its values into FileGroup::custom_keys; status.cpp appends them last,
to uploaded/missing/deleted rows, retention's shed rows, and the group_complete
marker, so a File group and its members are labelled the same way.

Collisions

Two kinds of key are dropped instead of written — in both cases from every row kind, not
just the ones that happen to carry the field, so a File and its group marker can never
disagree about what a name means:

Key	Behaviour
Names a field the Uploader computes (storage_type, size, remote_path, …)	The Uploader's value is kept; the key is reported in ProcessSummary::dropped_custom_keys and the Bridge logs a warning
name, id, robot_name	Not repeated and not reported — the rows already carry those values as group_name, robot_id and robot_name

The second row matters for the demos, which all configure
custom_key_str_list: ["robot_name", "id"]. Emitting a bare id would write a robot
identifier into whatever an id column happens to be — a serial primary key, for instance.

Scope

The columns still have to exist in the Destination. The PostgreSQL sink maps JSON keys onto
existing columns 1:1, so this makes the keys available on the File path; it does not
create schema.

Group-merged Records are unaffected: the Group node namespaces its members' fields under
their group_key, so those keys are no longer the Measurement's own labelling and the
parse is deliberately top-level only.

Tests

dc_bridge/test/uploader_test.cpp (4 new, asserting on the emitted rows):

• custom keys reach every file_status row (uploaded + deleted) and the group_complete marker
• a key naming a computed field never overwrites it and is reported in the summary
• robot_name/id are not repeated and not reported
• with no custom keys, both row kinds have exactly the key set they had before

dc_measurements/test/test_measurement_dummy.cpp (2 new): a Record with custom keys carries
the value and the declaration; without them the Record is untouched.

colcon test: dc_bridge 169/169, dc_measurements 295/295.

Docs

doc/src/dc/measurements.md — custom_key_str_list's row, plus a "Custom keys on Files"
section covering the collision rule and the ALTER TABLE caveat.

🤖 Generated with Claude Code

https://claude.ai/code/session_015oGKFjBD4WKxMHYv3iKXBo

[codecov]

Codecov Report

❌ Patch coverage is 81.57895% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.12%. Comparing base (0ed5a64) to head (68f2ce0).

Files with missing lines	Patch %	Lines
dc_bridge/src/bridge_node.cpp	0.00%	5 Missing ⚠️
dc_bridge/src/uploader/group.cpp	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy     #420      +/-   ##
==========================================
+ Coverage   70.80%   71.12%   +0.33%     
==========================================
  Files         114      114              
  Lines        7440     7478      +38     
==========================================
+ Hits         5267     5318      +51     
+ Misses       2173     2160      -13     

Flag	Coverage Δ
cpp-jazzy	71.12% <81.58%> (+0.33%)	⬆️

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