Make subdomain minimum length configurable via env var (#23209)

## What

Introduces a `SUBDOMAIN_MIN_LENGTH` environment variable (default `3`)
controlling the minimum number of characters allowed for a workspace
subdomain.

Until now the minimum was hardcoded (`3`), baked into the shared
`SUBDOMAIN_PATTERN` regex.

## How

- Added the `SUBDOMAIN_MIN_LENGTH` config variable (default `3`) in
`config-variables.ts`.
- Relaxed `SUBDOMAIN_PATTERN` in `twenty-shared` to validate format and
max length only, so the minimum length policy now lives with the caller
instead of being embedded in the regex.
- `isSubdomainValid` now takes a `minLength` argument (defaulting to
`3`) and enforces it explicitly.
- `SubdomainManagerService` reads `SUBDOMAIN_MIN_LENGTH` from config and
passes it to every validation call, making the server the authoritative
source.

## Scope

Server-side only. The frontend validation schema keeps its default
`.min(3)` UX check and is unchanged; the server remains the source of
truth for what subdomains are accepted.

## Tests

- Updated the shared `isValidTwentySubdomain` tests to reflect that the
pattern no longer enforces a minimum length.
- Added tests for the configurable minimum in
`is-subdomain-valid.util.spec.ts`.
- Updated the service spec config mock to return a numeric value for
`SUBDOMAIN_MIN_LENGTH`.

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packages/twenty-shared/src/utils/validation/__tests__/isValidTwentySubdomain.test.ts

@@ -16,7 +16,10 @@ describe('isValidTwentySubdomain', () => {
       expect(isValidTwentySubdomain('a-b-c-d-e')).toBe(true);
     });
 
-    it('should accept minimum length subdomains (3 characters)', () => {
+    it('should accept short subdomains regardless of minimum length', () => {
+      // Minimum length is enforced by the caller, not by the pattern
+      expect(isValidTwentySubdomain('a')).toBe(true);
+      expect(isValidTwentySubdomain('ab')).toBe(true);
       expect(isValidTwentySubdomain('abc')).toBe(true);
       expect(isValidTwentySubdomain('a1b')).toBe(true);
       expect(isValidTwentySubdomain('a-b')).toBe(true);
@@ -41,11 +44,6 @@ describe('isValidTwentySubdomain', () => {
       expect(isValidTwentySubdomain('')).toBe(false);
     });
 
-    it('should reject subdomains shorter than 3 characters', () => {
-      expect(isValidTwentySubdomain('a')).toBe(false);
-      expect(isValidTwentySubdomain('ab')).toBe(false);
-    });
-
     it('should reject subdomains longer than 30 characters', () => {
       const tooLong = 'a'.repeat(31);