feat(message): unify bulk template variables onto the shared {{name}} renderer (#69)

[rmyndharis]

Closes #69.

Problem

The gateway had two placeholder conventions for variable substitution:

• Message templates (send-template, stored templates) use double-brace {{name}} via the shared renderTemplate helper.
• Bulk send (send-bulk, messages[].variables) used a separate single-brace {name} renderer inside bulk-message.service.ts.

So the same idea — "substitute a variable" — needed different syntax depending on the endpoint, and {{name}} in bulk content was silently mangled to {value} by the single-brace regex.

Fix

Route bulk content through the shared renderTemplate, extended to accept both conventions via one ordered-alternation regex:

/\{\{\s*([\w.-]+)\s*\}\}|\{(\w+)\}/g

The double-brace branch is tried first at each position, so {{name}} is consumed as a unit and the legacy single-brace branch never sees its interior (no {{name}} → {value} mangling). The legacy branch keeps the historical {name} shape exactly (word chars, no whitespace), so existing single-brace bulk content renders bit-for-bit as before.

• {{name}} is now the canonical form, consistent across the gateway and documented.
• {name} is deprecated but still substituted for backward compatibility.

Tests

• New template-render.spec.ts: both syntaxes, mixed strings, the {{name}}-not-mangled disambiguation, missing-key-left-literal, whitespace, dotted/dashed keys, empty-string value, non-placeholder braces.
• New bulk tests: canonical {{name}} substitutes; legacy {name} still works.

Notes

• Updated the API spec bulk examples to {{name}} + a deprecation note on messages[].variables.
• One minor behavior refinement: an explicitly-provided empty-string variable now substitutes to '' (the old single-brace renderer treated '' as "missing" and left the placeholder literal). The shared hasOwnProperty semantics are the more correct behavior.

Full suite: 1096 passed / 96 suites; build + lint clean.