Narrow the broad blocks in ApiExecutor + topic_synth

[VGonPa]

Pre-existing finding surfaced by the silent-failure-hunter reviewer on PR #23.

Two production code paths catch every Exception and emit a single warn: line to stderr while continuing the loop:

• src/xbrain/executors/api.py:107-114 (ApiExecutor.enrich_items)
• src/xbrain/topic_synth.py:102-109 (synthesize_overviews_api)

A 100%-broken run currently looks like a no-op success. The CLI prints e.g. Topics sintetizados: 0/12 · 12 páginas escritas and exits 0 even when every single topic failed for the same reason.

Scope

• Narrow each catch to a concrete tuple: (anthropic.APIError, ValueError, json.JSONDecodeError, xbrain.validate.ValidationError).
• Count failures alongside successes; surface the totals in the stdout summary (e.g. Topics sintetizados: 0/12 · 12 fallidos).
• Raise a non-zero exit when failures == len(items) and no successes — a completely-failed batch is not a successful run.
• Keep partial-success behaviour: 11 successes + 1 failure should still write the 11 and exit 0 with a warning.

Tests

• Existing tests assert successful batches still succeed.
• New tests: monkeypatch the client to raise on all items; assert non-zero exit and a clear summary line.
• New tests: monkeypatch to raise on a subset; assert partial-success behaviour.

Out of scope

• Structured logging migration (the print(..., file=sys.stderr) pattern is consistent with the rest of the project; that is a separate refactor).
• The _handle_cli_errors swallow in cli.py (similar shape, separate issue if needed).

[VGonPa]

Closed by #31 (merged to develop).