chore: relicense BUSL-1.1 → Apache 2.0 for 1.6.0 (#651)

* chore: relicense BUSL-1.1 -> Apache 2.0 for 1.6.0

Flips every license artifact in the tree; 1.5.x and earlier remain
BUSL-1.1 per their release-time LICENSE files. Contributor consent
record: #548 (rationale: #546).

- LICENSE: canonical Apache 2.0 text
- NOTICE: new; copyright line + pointer to THIRD-PARTY-NOTICES
- pyproject.toml: SPDX expression + explicit license-files trio
- Dockerfile: COPY the license trio (hatchling needs them at build)
- THIRD-PARTY-NOTICES: BUSL line reworded; bundled-version drift
  fixed (KaTeX 0.17.0, Mermaid 11.15.0, hls.js 1.6.16)
- README badge + License section, CONTRIBUTING inbound-license line,
  TS SDK package(+lock), example pyproject
- docs/pgbouncer.md: drop stray ':' introduced in #353

* docs: add CONTRIBUTORS.md

* chore: drop LICENSE leading blank line

The apache.org LICENSE-2.0.txt begins with a newline; the SPDX
canonical text and GitHub license templates do not. Use the
conventional form — detection is whitespace-normalized either way.

sdk/typescript/package-lock.json

@@ -7,7 +7,7 @@
     "": {
       "name": "@turnstone/sdk",
       "version": "0.4.0",
-      "license": "BUSL-1.1",
+      "license": "Apache-2.0",
       "devDependencies": {
         "typescript": "^6.0.0",
         "vitest": "^4.1"

sdk/typescript/package.json

@@ -30,7 +30,7 @@
     "sdk",
     "client"
   ],
-  "license": "BUSL-1.1",
+  "license": "Apache-2.0",
   "devDependencies": {
     "typescript": "^6.0.0",
     "vitest": "^4.1"