Closes #5329

Summary

Audit: docs/issues/api-error-generic-missing-fluent-key.md (Severity: Critical).

41+ HTTP 500 handlers in crates/librefang-api/src/routes/{agents,tools_sessions}.rs
build their response body as:

t_args("api-error-generic", &[("error", &e.to_string())])

But api-error-generic was never defined in any locale's errors.ftl
(grep -rn 'api-error-generic\s*=' against
crates/librefang-types/locales/{en,ja,zh-CN,de,fr,es}/errors.ftl → 0 hits).
The fallback at i18n.rs:163-164 returns the literal key on a missing
lookup, so the {$error} interpolation never ran — every one of those
5xx responses surfaced {"error": "api-error-generic"} to the client
with the actual e.to_string() silently dropped.

This swallowed the diagnostic context behind every 500 in agents +
tools_sessions routes — including the bug classes
agent-not-found-returns-500, rusqlite-errors-leak, and the
agent_clone 500-on-AgentAlreadyExists case folded into the same
audit item.

Fix

Per the audit doc's stopgap: define the key in every locale.

# en
api-error-generic = { $error }

# ja
api-error-generic = エラー: { $error }

# zh-CN
api-error-generic = 错误: { $error }

# de
api-error-generic = Fehler: { $error }

# fr
api-error-generic = Erreur: { $error }

# es
api-error-generic = Error: { $error }

English uses bare { $error } (the underlying error strings are
already in English). The five other locales prefix with a localized
"Error / エラー / 错误 / Fehler / Erreur / Error: " so the rendered
string differs across bundles — the per-locale assertion in
all_languages_have_same_keys requires non-English bundles emit a
distinct rendered output, which a bare-shared pattern would not.

The long-term remediation (route every 500 through a typed
MemoryRouteError-style helper per routes/memory.rs:198-215) is
tracked separately and intentionally out of scope here.

Regression guards

• api-error-generic appended to the key list in
  all_languages_have_same_keys. A new locale or a stale
  errors.ftl cannot silently regress this without a CI failure.
• New dedicated test api_error_generic_interpolates_underlying_error
  exercises every SUPPORTED_LANGUAGES entry with a sample $error
  and asserts (a) the rendered body is not the literal key and
  (b) the underlying error string is contained verbatim.

Verification

cargo test -p librefang-types --lib i18n
# 22 passed (incl. 1 new), 0 failed

Test plan

• CI Test / Unit (lib+bin) green
• Hit a known 500-emitting route (e.g. malformed agent payload),
  confirm the response body contains the real error text instead of
  "api-error-generic"
• Switch dashboard locale to zh-CN / ja, confirm the localized
  prefix appears